.-.

Frank M. Chapman

FOR THE PEOPLE

FOR EDVCATION

FOR SCIENCE

LIBRARY

OF

THE AMERICAN MUSEUM

OF

NATURAL HISTORY

*?

THE FLIGHT OF BIRDS

THE

FLIGHT OF BIRDS

By F. W. HEADLEY, M.B.O.U.

Author of " The Structure and Life of Birds "
" Life and Evolution" Sec.

WITH SIXTEEN PLATES
AND MANY TEXT-FIGURES

WITHERBY & CO.

326 HIGH HOLBORN LONDON

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IS- Mi>0<(.2fc-3w~O

PREFACE.

The flight of birds cannot but be of interest to
men who sail the air on their biplanes or monoplanes,
for the bird is, as yet at any rate, peerless among
aviators, and in describing his methods I have kept
in view the methods and the difficulties of those
who are striving to rival him. It is possible, there-
fore, that this little book may find readers among
those who not only study flight but fly. But, since
I have tried as far as possible to avoid technical
terms and make it easily intelligible, I hope it may
appeal to the larger class who, whether scientific
ornithologists or not, take a delight in birds and
their doings.

I have to thank many friends for their help ; Mr.
J. A. Tregelles for three drawings which can speak
for themselves (the frontispiece and figures 13 and 14)
and also for reading a typewritten copy of the
book : Mr. C. W. Adams for reading the first five
chapters : four Haileyburians past or present for
drawings and a photograph ; W. T. Hichens for
figures 18-23 ; G. G. Nanson for the diagrams ;

G. H. G. Perry for figure 27 ; A. P. "Whitehead for
Plate xrv, d. Mr. P. Disney has been kind enough
to read the proofs. In addition I have to thank
Mr. P. C. Gilson for some valuable criticisms, Mr.
F. H. Jeffery for help in some small experiments :
Messrs. Duckworth have very kindly allowed me
to reproduce several of the illustrations of my Life
and Evolution. The photographs (with the one
exception I have mentioned) are my own. Most of
them are now published for the first time, but
several have appeared in Britisli Birds, and three
in the Journal of the Aeronautical Society. The
Editors of British Birds and the Secretary of the
Aeronautical Society kindly permit me to use them.

F. W. HEADLEY.

Haileybury,

March, 1912.

CONTENTS.

CHAPTER I.

Gliding. pagb

Resistance of Air — Lift and Drift — Curve of Wings — Area

of Supporting Surface . . . . . . . . . . 1

CHAPTER II.

Stability.

Centre of Gravity — Points that make for Automatic Stability

— Voluntary Adjustments . . . . . . . . 23

CHAPTER III.

Motive Power.
Leverage — Propulsion — Phases of the Wing-stroke . . 38

CHAPTER IV.

Starting.

The Preliminary Jump — Loss of Altitude between Wing-
strokes — The Wing's Freedom to Rotate — Aeroplanes
— Muscles — Big Birds and Small . . . . 48

CHAPTER V.

Steering.
A Variety of Methods — Good Steerers and Bad . . 59

CHAPTER VI.

Stopping and Alighting . . . . 64

CHAPTER VII.

The Machinery of Flight.

The Breastbone and the Connected Bones — Muscles and
Quality of Muscle — The Scaffolding of the Wing — Pneu-
matic Bones — Stiffness of Wing — Expanse of Bone —
The Spreading of the Wing — Structure of a Flight-
Feather — Moulting — Legs . . . . . . 67

CHAPTER VIII.

Varieties of Wing and of Flight.

Curve — Narrow and Broad Wings — Styles of Flight — Flight

in Flocks— The Whir of Wings 91

CHAPTER IX.

Pace and Last.

Experiments and Observations — Wind — Velocity of Migra-
tory Flights — Endurance . . . . . . . . 103

CHAPTER X.

Wind and Flight.

Rising — Flight with the Wind — Undulating Flight
without Movement of the Wings — Advance in a Direct
Line without Movement of Wing — Advance Sideways
in a Direct Line — Soaring — Soaring in a Horizontal
Wind Impossible.. .. .. .. .. .. 120

CHAPTER XL

Some Accessories.

Digestion — Circulation, Breathing, Temperature — Repair

of the Machine — Call-notes and Song . . . . . . 154

LIST OF PLATES.

I. — Eagles soaring . . . . . . . . Frontispiece

Facing
page
II. — Photographs of Pigeons and Gull, showing right

and left wings giving different strokes . . . . 30

III. — Photographs of — A : Pigeon using tail for mainten-
ance of equilibrium. B : Pigeon gliding. C :
Gulls gliding 36

IV., V., and VI. — Photographs of Pigeons showing the

phases of the wing-stroke in series . . 40, 42, 45

VII. — Photographs of Gulls flying . . . . . . 46

VIII. — A and B : Photographs of Pigeons starting to fly.
C : of Pigeon rising with body steeply inclined.
D : of Herring-Gulls starting . . . . . . 49

IX. — Photographs showing different methods of steer-
ing — A and B : of Gulls. C and D : of Pigeons 60

X. — A and B : Photographs of Pigeons alighting. C :

of Pigeon checking speed . . . . . . 64

XI. — A : Photograph of wing, showing elastic liga-
ment. B : of flight-feathers and tail-feather . . 85

XII. — Drawing by I. C. Maclean (from Life and Evolu-
tion), showing minute structure of flight -feather 86

XIII., XIV., XV.— Photographs of different types of

wing 91, 92, 94

XVI. — Photographs of Gulls with motionless wings fol-
lowing steamer . . . . . . . . . . 136

IX

LIST OF FIGURES IN TEXT.

Page
1. — Glass balls, an illustration of Sir I. Newton's experi-
ment . . . . . . . . . . . . . . 3

2. — Diagram illustrating gliding . . . . . . . . 6

3. — Parallelogram of forces . . . . . . . . . . 7

4. — Diagram and drawing of boat tacking . . . . 8

5. — Illustrating kite-flying . . . . . . . . . . 9

6. — Showing the proportion of resistance to support . . 10

7. — Further illustrating the subject of resistance and

support . . . . . . . . . . . . . . 11

8. — Planes set at different angles to the horizon . . . . 12

9. — Showing the advantage of a curved surface . . 16

10. — Two cubes 19

11. — Showing the disadvantage of an excessive curve .. 25

12. — Showing the shifting of the centre of pressure . . 26

13. — Swift gliding with wings fully extended . . . . 35

14. — House-Martin gliding with wings partly flexed . . 35

15. — Showing the velocity with which the extremity of the

wing moves . . . . . . . . . . . . 39

16. — Showing how the downstroke propels as well as lifts 42

17.— (After Marey) Gulls flying 43

18. — Breastbones of Guillemot and Falcon . . . . . . 68

19. — Breastbone and connected bones of Adjutant . . 69

20.— Clavicles of Tern and Eagle 70

21. — Humerus of Eagle . . . . . . . . . . 71

22. — Humerus of (a) Skua ; (6) Hornbill ; (c) Sea-Eagle.

Drawn to scale . . . . . . . . . . . . 78

23. — Skeleton of wing of Adjutant . . . . . . . . 80

24. — (After Alix) showing elastic ligament, anterior wing-
membrane, eto . . . . . . . . . . . . 82

25. — Illustrating advance with motionless wings at right

angles to the wind . . . . . . . . . . 131

26. — Illustrating advance in the teeth of the wind with

wings held rigid . . . . . . . . . . 136

27. — Gull gliding sideways with wings held rigid . . 140

THE FLIGHT OF BIRDS.

CHAPTER I.
GLIDING.

RESISTANCE OF AIR LIFT AND DRIFT CURVE OF WINGS

AREA OF SUPPORTING SURFACE.

Not long ago the sight of a sparrow on the wing,
or even the sight of a lark rising in air and singing
as he rose, excited but little interest or wonder in
the mind of the average man. It came natural
to birds to fly, and to fly so perfectly that they
concealed their art. If they had had difficulty in
flying, there would have been matter for astonish-
ment. But why waste any wonder on the easy
flight of a lark or a sparrow ? Such familiar things
were taken for granted and seemed to call for no
explanation. As soon, however, as men began to
emulate birds — gliding downwards first from some
elevated position and, later, by means of powerful
engines, rising from the ground — then there was
less disposition to take the bird's flight as a matter
of course. The bird too, it was felt, had once
been a beginner. He too had had difficult problems
to solve and had solved them long ago, the very
perfection of his methods making it difficult to

2 THE FLIGHT OF BIRDS

understand fully what his methods are. But what-
ever the difficulty of learning from him, the bird had
certainly much to teach. Such a past master in
the art of flying must needs be able to give some
hints to the man whose ambition it was to discover
the ideal design for an aeroplane.

And so the question " How does a bird fly ? "
became one that had an interest not only for
ornithologists. Those who gave it any thought
soon found that it involved a number of problems.
First comes the question how the yielding air can
support a heavy body when gravity is tugging it
downwards. Then, of no less practical interest
is the question, how the bird maintains his equi-
librium, or recovers it if for the moment he happens
to lose it. How does he move his wings so that
they may at once propel and support him ? How
does he steer ? How is it that the small bird is
able to start so easily from the level ground ?
What of the easier but important problem of alighting
without jar ? How is it that the bird, big or small,
is able to treat with contempt the sudden gusts
and eddies that the boldest aviator fears ? Does
the bird ever gain advantage from the unequal
velocity of the wind ? Does he search for up-currents
and get them to lift him ?

Resistance of Air.

The first of these problems was solved by Sir
Isaac Newton. By means of an experiment, that
may well astonish us if we bear in mind how
imperfect were the appliances that he had at his
disposal, he demonstrated the peculiar property of

GLIDING 3

air that makes flight a possibility. The resistance
of air to a moving body may be little, may be
great. That it may sometimes be considerable
many a bicyclist has found out to his cost when
he has tried to double and redouble his speed.
There comes at last a time when the " yielding
air " almost refuses to yield. In fact the resistance
it offers to a body moving through it increases as
the square of the velocity. Of the ingenious experi-
ment by which Newton proved this I must give
a brief account. He took glass globes of equal
size but unequal weights, corresponding to the
figures 1, 4, 9, 16. These he let fall from the dome
of St. Paul's and measured the velocities when
they had settled down to a uniform pace. Since
there was no gain or loss of velocity, the resistance
of the air must have been in each case equal
to the weight of the falling globe. But it

turned out that the relative velocities corresponded
to the figures 1, 2, 3, 4, whereas the weights of the
globes are represented by the squares of these
numbers, viz. 1, 4, 9, 16. From this he concluded
that the resistance of the air increases as the square
of the velocity. Recent experiments have shown
that Newton's law is not absolutely accurate. Up

B 2

4 THE FLIGHT OF BIRDS

to 10 metres per second (about 22 miles per hour)
the increase in resistance is rather less than the
square of the velocity, Avhereas for velocities greater
than 10 metres per second the resistance increases
at a still more rapid rate.*

But to resist, to give effective support, the air
must be in proper condition. If it has just been
pounded and battered, it is useless to trust to it.
And so perpetual forward movement to ever fresh
fields is necessary. A bird cannot mark time in
the air. It is true that one often sees a Kestrel
Hawk hovering, apparently without any forward
motion, his wings, one might imagine, pounding
the same air over and over again. But there is
good reason to believe that the Kestrel never hovers
except when there is a fair breeze to bring fresh
unbattered air to his wings. If a long string be
tied to a Pigeon's leg, he will fly perfectly well till
he reaches the end of his tether. But as soon as he
feels the pull of the string he will drop to the
ground. When a flock of birds are travelling across
the sky, it is easy to see that not one of them puts
himself immediately behind any other. Were any
individual to do so, he would not have at his disposal
the fresh undilapidated columns of air that are
essential, and moreover he would feel all the back-
wash from the bird in front. At least one aviator
has lost his life through getting into the wash of
an aeroplane that was a little ahead of him. In
building a biplane the question of finding air that
has not been tumbled is one that cannot be neglected.
A writer who appears to speak with authority says

* Marey, Vol des Oiseaux, p; 218.

GLIDING 5

that its two surfaces must have a space of four
feet between them, or else they will interfere with
one another ; one will tumble the other's air. It
is true that I have seen a triplane with its " planes "
at a distance from one another of three feet only.
But whatever the minimum interspace, it must be
considerable. And yet when birds are flying low
over water they do not ruffle its surface. Pelicans
flying over the Nile, not more than a foot from the
water, so a good observer says, left the surface
undisturbed.* Still, I cannot help thinking that
the space between the birds and the water was
under-estimated. At any rate a pigeon's first
strong wing-strokes when he rises scatter straws,
dust, and feathers from the ground where he takes off.
Enough has now been said to show how effective
is the support that the air can give to a rapidly-
moving body, and how essential it is that the bird
or the aeroplane should be unceasingly moving
onward to columns of air that are fresh and
undisturbed.

Lift and Drift.

It is probable that the power of flight was first
attained by terrestrial birds which jumped from
tree to tree, their wings aiding them to float through
the air. If this floating through the air had been
their crowning achievement, they would have done
nothing remarkable, for mere gliding is not flight.
Since, however, gliding seems to have been their
first attainment, just as it has been with human
flyers, I shall begin by investigating this compara-
tively simple performance. At the outset there

* Quoted by Marey, Vol des Oiseaux, p. 30.

6 THE FLIGHT OF BIRDS

is a very elementary matter to be made clear. Let
us picture the bird to ourselves as gliding in an
exactly horizontal direction. The resistance of the
air to his forward progress is equivalent to a
horizontal wind blowing against him, and what
is wanted is support, to counteract the downward
pull of gravity. From a horizontal wind he must
somehow get this support. The question presents
no difficulty to anyone who has studied elementary
mechanics, and in the application of mathematical
principles the bird is a wonderful proficient. He
inclines his aeroplane (his expanded wings and his

W

Fig.

body) slightly upward, and the result is that the
air supports him more than it resists him. It acts
at right angles to the plane that he opposes to it.

In fig. 2, b represents the gliding bird, w the
rush of air against his expanded surface. If the air
is still, there will, nevertheless, be the wind due to
the bird's own velocity. W, at right angles to the
bird, shows the direction in which the wind acts ;
its lifting power, when the bird inclines himself
thus, being greater than its resistance. The action
of the air at right angles to a plane moving through
it, is illustrated by an experiment sometimes made
by very unscientific persons. A stone is thrown

GLIDING 7

slanting- wise at a window — thrown by a dexterous,
mischievous urchin standing far off beneath the
wall of the house — and the fragments of glass as
they go flying into the room make a right angle
with the plane of the window.

The air, then, striking against the plane presented
by the bird acts at right angles to it, and there
comes into play a resolution of one force into two.
This introduces what is known as the parallelogram
of forces.

There is a force acting along a b (fig. 3), and if
resistance is in this direction it may resolve itself

C ^.

Fig. 3.

into two forces represented in magnitude and direc-
tion by the lines c b, d b. Take the case of a boat
tacking. The wind acts at right angles to the sail,
but the boat refuses to move much in that direction
(i.e. broadside on) ; she makes only a little leeway.
The force of the wind, therefore, acting towards x
(fig. 4), is broken up into two forces acting towards
d and l, and that towards l, as I have said, does not
count for much, since the boat will not readily move
broadside on. So there is much headway and a
little leeway. The principle at work is the same
when a bird is gliding horizontally. His body is
inclined slightly upward. The force of the air acts

8

THE FLIGHT OF BIRDS

at right angles to the surface he presents to it.
And this force is broken up into two, the one
supporting him, the other tending to drive him
backward, or, as it is now briefly and clearly put,
the force is resolved into lift and drift.

Fig. 4.

Boat tacking. — w, the wind which acts at right angles to sl
(the sail), towards x. The force is broken up into two forces,
acting towards d and l.

A paper kite supplies us with another and perhaps
still more apt illustration. If the air is still, the
kite-flyer may supply a force by running with the
string ; if there is a wind, he has merely to hold
the string. Let us imagine that the wind is blowing
horizontally. It will, nevertheless, lift the kite, as
long as the string is held firmly. It strikes against
the oblique surface which the kite presents, and

GLIDING 9

acts at right angles to it. Drift is prevented by the
firmly-held string, and lift is the sole result (fig. 5).

It is only when there is resistance that there is
any force to be thus resolved into two. If the
string breaks, gravity at once begins to pull the
kite to the ground.

We have now a further question to investigate
and, to simplify it, we must consider not the gliding
bird, but a flat plane set at a slight upward incline
and driven horizontally through the air. A flat
plane, having none of the curves and concavities
of the bird's wing, is far inferior for purposes of
flight, but its simplicity recommends it when the

W

Fig. 5.

To Illustrate Kite-Flying.

w, horizontal wind blowing against kite, k t. w' (at right angles
to k t), line along which the force of the wind acts.

object is to explain elementary principles. Set at
an incline and moving horizontally it will tend to
rise. The resistance of the air is equivalent to a
wind blowing against it. The wind would act in

10 THE FLIGHT OF BIRDS

a direction at right angles to the plane, and the
force so acting is, of course, resolved into two, one
tending to raise the plane, the other resisting its
horizontal progress. The question, then, which we
wish to decide is : In what proportion is the force
of the wind divided between the two components,
between lift and drift ? Now, supposing that b d
(fig. 6) represents the plane set with an upward incline
and driven horizontally through the air, it can be
shown that line d c represents the resistance of the

Fig. 6.

air to its onward progress and b c, a much longer
line, the support given by the air. In fact, when
the plane is inclined but slightly upward the support
it gets from the air is far greater than the resistance,
a fact that can be proved by experiment. The
mathematical proof that d c represents the drift
(or resistance), and b c the lift, I give in fig. 7.

It is now apparent that, as the angle of inclination
to the horizon is more and more reduced, the pro-
portion of lift to drift becomes greater and greater.
Why not, then, reduce the angle till the resistance
of the air to horizontal progress becomes a negli-
gible quantity ? But obviously there is a limit
to the process. If the plane has so slight an incline
that it is almost horizontal, the air will offer but
little resistance, and however big a proportion of
this we may allot to lift and however small a one

GLIDING

11

to drift, yet the actual amount of lift will be but
small. If the strongest man among the survivors

w

Fig. 7.

b d is the plane, w the wind blowing horizontally against it.
b c is the cosine of the angle at b, d c the sine of the angle. Pro-
duce c d to e, making d e = b c. From d draw d p at right
angles to b d. From e draw e f at right angles to d e. (This
decides the length of the line d f). Draw f g parallel to e d
and d g parallel to f e. Let d f represent the force of the wind
acting at right angles to b d. It can be resolved into two forces,
f e and fg(=ed). e d we know= b c, and it can be shown
that f e = d c.

In the triangles d e f, b c d, b c = d e, and the angles at e
and c are each right angles. If we could prove a second angle
= a second angle, the triangles would be equal in every respect.
Now the angles at d together = two right angles, and the angle
b d f is a right angle. Therefore b d c with f d e makes one
right angle. But d f e with f d e makes one right angle. There-
fore angle d f e = angle b d c. The triangles then are equal,
and the two sides f e and d e = respectively d c and b c. But
f e represents drift and d e lift. Therefore d c, the sine of the
angle at b, represents the drift, while b c, the cosine, represents
the lift.

of a starving ship's crew is able to take for himself
nineteen-twentieths of the last biscuit, nevertheless
he gets but a poor meal ; and it is obvious that if

12 THE FLIGHT OF BIRDS

the plane were not inclined at all, but presented
its edge to the air, there would be practically no
support. There is, therefore, beyond all dispute
a limit somewhere to the possible reduction in the
incline of the aeroplane. The question is where,
for practical purposes, that limit comes in. Newton
formulated a law with regard to this, a law which
is now quoted only to be condemned, and some-
times quoted with expressions of contempt for
him and mathematicians in general.* Newton
held that the resistance of the air increases as the
square of the sine of the angle of inclination.

Fig. 8.

Thus, if we take angles of 5°, 10°, 20°, the resistance
would increase, from 25 to 100, to 400. Instead of
being grateful to Newton for his great contribution
to our understanding of flight, his discovery that
the resistance of the air increases as the square
of the velocity of bodies moving through it, some
writers have depreciated him and his work because
he has come to a wrong conclusion on this further
question. As a matter of fact the resistance of
the air varies as the angle, i.e. as its sine, not as
the square of its sine. Therefore, as you diminish
the angle, you still have a considerable amount of
resistance, and it is divided up largely in favour

* See Sir H. Maxim's Artificial and Natural Flight, pp. 2-G.
The question is well dealt with in Prof. Langley's Experiments
in Aero-dynamics ; see especially pp. 24 and 25.

GLIDING 13

of support, of lift as opposed to drift. If Newton's
law held good, if the angle of inclination were more
and more reduced till it amounted only to, say, 5°,
then the resistance offered by the air would be too
small to be worth dividing up between lift and drift.
Even the lion's share would be worth next to
nothing. But experiment shows that an aeroplane
set at an angle of 5° can, if it travels fast, find
support in the air.

But, although as we reduce the angle of inclina-
tion the resistance of the air does not diminish at
the rapid rate that Newton imagined, nevertheless
there must obviously be a point beyond which the
fining down of the angle cannot go, since the air
will at last cease to give the required support.
But before we reach the lowest possible limit another
factor comes in which checks us as we are making
successive reductions. As we continue to cut
down the angle there comes at last a point at which
the question of friction obtrudes itself in very
unpleasant fashion. Imagine the aeroplane driven
through the air at a very minute angle. If it is
to find support, it must travel at a very great pace,
else the resistance of the air will be too small.
With every diminution of the angle there must
be an increase of pace, and it might be thought
that, if only the pace were increased sufficiently
to make up for the diminution of the angle, all
would go well. Professor Langley made some
most valuable experiments which showed the great
advantage of a small angle of inclination, and,
emboldened by this great and important discovery,
he proceeded to frame a formula and to speak

14 THE FLIGHT OF BIRDS

of a law. By experiment he had discovered an
indubitable principle, but subsequent experiments
have discovered an equally indubitable fact, which
at a certain point interferes with its operation.
The angle cannot be reduced below, approximately,
5° without bad results. At a less angle, with the
necessary increase of pace, the friction of the plane
against the air increases so rapidly that, so far from
there being any gain from the further reduction
of the inclination, there is an actual loss.*

On this subject a writer in Flight has some very
interesting calculations, founded partly, it is true,
on theory, and requiring further verification by
experiment, but probably representing the facts
without any considerable deviation. f Indeed experi-
ment has already proved his main thesis, so that
it is only detail that requires further testing. He
imagines aeroplanes having the ideal camber or
curve for their planes, a large curve if they are to
travel slowly at a great angle of inclination, a slight
curve if they are to travel fast. They are to carry
a weight of 100 lb. Let us imagine them driven by
3 horse-power at the angles 30°, 25°, 20°, 15°, 10°, 5°.
The velocity will increase rapidly as the angle is
reduced. At 30° it will be 38 miles per hour.
With the successive reductions of the angle it will
increase to 47, 60, 78, 106, 134. The pace at 5°
and even at 10° is greater than most people would
wish to travel at, so the power applied might with
advantage be reduced. Two horse-power at 10°

♦See The Aero Manual, p. 21 ; Flight, July 9, 1910; and
Langley's Aero-dynamics, p. 37.

t Loc. cit.

GLIDING 15

and 5° respectively would mean 70 and 90 miles
per hour. Even 1 horse-power, with an angle of
5° gives a velocity of 43 ; in other words, 1 horse-
power, if the plane is set at an angle of 5°, is more
effective than 3 horse-power when the angle is 30°.
In fact, if you have a good aeroplane and the skill
to use it well, you require less power to fly fast than
to fly slow. But if you reduce the angle of inclina-
tion below 5° you find that the tables are turned
upon you. So far from economising power, you
would have to use it in lavish style, to overcome
friction.

Curve of Wings.

We must now turn to the question of curves.
The curved surface is undoubtedly superior to the
flat. A toy paper glider, being a mere feather-
weight, can dispense with curves, but without
curved surfaces — cleverly designed ones too —
aviation would be out of the question. An aeroplane
presents concave surfaces to the air ; its " planes "
curve from front to back, and a bird's wings have
concavities that are probably better adapted for
flight than anything that human ingenuity has
designed. On this subject even an umbrella can
tell us something. When its ample concavity is
turned towards the wind its efficiency in catching
and holding the air is so great that it speedily
becomes a wreck of wires. Lilienthal, a distin-
guished flight pioneer, whose experiments in gliding
did much to make aviation practicable, fully ap-
preciated the value of the curved surface. Some-
times when he was carrying his glider to a little

16

THE FLIGHT OF BIRDS

hillock, his jumping-off place, outside Berlin, he
was cheered and emboldened by the way the wind
would catch his glider's well-designed concavities
and nearly lift him from the ground. When he
reached his little hillock he would jump from it
and make glides of 150 yards and more.

Lilienthal found that the air that met his glider's
concave surface did not act at right angles to the
chord of the arc (fig. 9), along f, but along a
line occupying somewhat the position of f'.

Fia. 9.
The arrow shows the direction in which the glider is travelling.

Experiments made by Mr. Wilbur Wright and
his brother have shown that Lilienthal had called
attention to a principle on which the aviator can
base his calculations. In short, with a properly
rounded surface the lift is greater and the drift is
less than with a flat one. In order to obtain the
maximum gain, the best possible curve must be
discovered, and this can only be done by experiment.
The writer in Flight already quoted is of opinion
that each velocity has its appropriate curve. An
aeroplane that is to be a racer should, he maintains,
have its surfaces much less curved than a slowly-
flying one, and undoubtedly the wings of birds
suggest that he has enunciated a true principle.

GLIDING 17

Very fast flyers, such as the Swift, have very little
concavity. There is another fact, too, that points
to the same conclusion : in a bird's wing the part
that is near the body is the most concave, while
towards the extremity it becomes nearly flat, in
some cases quite flat. Now the extremity of the
wing, when a stroke is given, moves with great
velocity forward as well as downward, the near
part comparatively slowly. Thus birds seem to be
exponents of the principle that the curve should
vary inversely as the speed. And here I must
point out a great advantage the bird has over his
upstart rival, the aeroplane. The aeroplane is
rigid, with at most the rear edge only of its plane
flexible. If the principle to which I have called
attention is sound, it must be built either with a
curve appropriate to great speed or with one which
suits comparatively slow travelling. It cannot
be varied during flight according as the pace is
varied. But when the bird takes a very hard
stroke, the front-to-back curve of his wings, which
is mainly the curve of the feathers, is much reduced,
so that by plying his wings with great vigour he to
some extent modifies his configuration and adapts
it to his high velocity. (For birds in rapid flight,
see Pis. iv-vn.)

It is always well to ask " Why is it so ? " when-
ever experiment discovers a fact. Why, then,
with a curved surface is the lift more and the drift
less ? It is very difficult to see how the air does
its work, and theory on such a subject may prove
to be only random guesswork. But the lift would
seem to be greater from the simple fact that the

18 THE FLIGHT OF BIRDS

air cannot so easily escape. When the moving
surface is a plane, the air is unconfined and the
force is dissipated. The reduction of the drift
(i.e. of the resistance to the forward movement of
the surface) may be illustrated by the passage of
a boat, built on good lines, through the water. The
resistance of the water to the bow is balanced, or
nearly so, by the shove given to the stern by the
water closing behind it, so that there is little, except
friction, to retard the boat. In the same way the air
closes upon the hinder part of the curved surfaces
of an aeroplane ; there is no region of " dead air "
behind it. The matter is well explained in a little
book on Model Flying Machines (pp. 19 and 20)
by Mr. W. G. Aston. There is a region of " dead
air " behind a flat surface when it is given an upward
incline and is driven forward horizontally. The
amount of " dead air " is the measure of the amount
of resistance.

Area of Supporting Surface.

We can hardly leave the subject of gliding without
touching on this important question : What area
of supporting surface is required for, say, one pound
weight ? Obviously no hard and fast rule can be
laid down. Since it is the front part of the plane
on which the wind mainly impinges, the back part
is of less importance ; indeed it may be quite
superfluous, a useless encumbrance, if the breadth
from front to back is excessive. Besides this, as
I shall show, we cannot frame a formula that will
apply equally to big aeroplanes and small, to the
big bird and the small bird. The Aero Manual

GLIDING

19

says, " Nearly one square foot for each pound
weight." But it has in view big machines only.
When we go to the various creatures that fly, weigh
them and measure their surfaces, we get most
diverse results. The big flyers, we find, have small
wings, the small flyers big ones, if difference in weight
is allowed for. Compared with a gnat or a butterfly
a Stork has a very small supporting surface, a small
one even when he is compared with a Swallow.
When M. de Lucy discovered these facts and pub-
lished them the astonishment was great. Here
are some of his figures : —

Gnat

Surface per lib.
sq. yds. ft.
..4 6

weight,
ins.
105

Butterfly . .
Swallow . .

.. 3

..

8
4

87
18

Pigeon
Stork

..
..

1

14
122*

^

/

/

Fig. 10.

But before long mathematicians hit upon a plan
by which they were able (or thought they were able)
to rob the figures of their startling character, and

* See Marey, Animal Mechanism, p. 222, and Pettigrew, Animal
Locomotion, p. 133.

C 2

20 THE FLIGHT OF BIRDS

put big and small birds in this respect more or
less on a level. They appealed to the elementary
principles of geometry. If you take two cubes,
a side of one of which is twice the length of the
other, the larger one is in bulk eight times as great
as the smaller one, but its surface area is only four
times as great. This holds true of other figures
of three dimensions that are not cubes. Magnify
a bird till it is eight times its former bulk and you
will only have multiplied its surface area by four.
In order, then, to compare bird with bird correctly,
you should take (so say these theorists), the cube
root of its weight (for the weight is practically the
bulk, i.e. three dimensions multiplied together)
and the square root of its surface area (since that
is two dimensions multiplied together). When we
adopt this method we find the preponderance of
the small bird in point of wing-area per pound
weight not so very great. And we may, if we are so
constituted, derive a certain comfort from feeling
that we are following out geometrical principles.
But these principles have, it must be owned, in the
case of some species been widely departed from.
What are we to make of the legs and neck of the
Flamingo ? If any small bird of ordinary build
were symmetrically enlarged, should we ever arrive
at elongations so enormous ? It is, of course, true
that if the bulk of a small bird were multiplied
many times, and the area (not the bulk) of his wings
increased in proportion, he would have a greater
expanse of wing than his muscles could possibly
work. If the Stork had a wing-area as great in
proportion to his weight as the Swallow, then

GLIDING 21

(taking his weight as four pounds and four-fifths)
his wings would together measure twenty square
feet ! This would be a monstrous acreage of wing
to raise and lower. But when we have pronounced
it monstrous, we have still to answer the question
why it is that the big flyer requires, in proportion
to his weight, a comparatively very small support-
ing surface ?

Let us imagine the Swallow supplied with wing-
area at no more liberal rate per pound weight than
that at which the Stork is supplied. Then, taking
the Swallow's weight to be about five-sevenths of
an ounce, he would have five square inches of
wing-surface — two and a half on either side — a
miserably poor allowance. A wing so small would
be largely made up of margin, and the air would
escape at the edges. The gnat has over four and
a half square yards of wing for one pound weight.
His actual allowance for his almost imponderable
insignificance is considerable, but if we make pro-
vision for him at the rate at which the Stork is
supplied, his wing-surface becomes a mere point,
"without parts and without magnitude," to quote
Euclid's familiar definition. The air would offer
no resistance to so near an approximation to the
theoretical point. Here, no doubt, we are getting
at the main fact that explains the comparatively
small size (when weight is allowed for) of the wings
of the great flyers. Since the wing-surface is much
larger absolutely the air does not escape so easily
at the margins ; each square inch is more effective,
since there is less waste. Later on I shall make
a further comparison of big birds and small, but

22 THE FLIGHT OF BIRDS

one important difference and its explanation have,
I hope, been made clear (see Chap. iv).

It will be more convenient to consider the question
of upward and downward gliding in the next
chapter.

CHAPTER II.
STABILITY.

CENTEE OF GRAVITY — POINTS THAT MAKE FOR AUTOMATIC
STABILITY VOLUNTARY ADJUSTMENTS .

There are two things that we must carefully
distinguish — stability and the maintenance of equi-
librium. An aeroplane, when it has once completely
lost its balance, cannot recover it, though some
swaying or pitching can, no doubt, be corrected. The
stability of a bird is a very different thing from
the aviator's careful maintenance of equilibrium. A
very strong and sudden gust may throw the bird on
his side, or even on his back, and yet he will very
quickly right himself.

Centre of Gravity.

But we want to know whether a bird in ordinary
flight, when there are no very sudden gusts, is auto-
matically stable, or whether he has to be perpetually
making small adjustments. It is sometimes main-
tained that a bird need take no trouble about the
question of balance, since his centre of gravity is
low down. The great flight muscles, which are
massed upon the breast, form a great part of his
weight. I have weighed the three pairs of breast
muscles of two Wood-Pigeons and those of two

24 THE FLIGHT OF BIRDS

domestic Pigeons, and found that they account for
about one-fifth, or even more than one-fifth, of the
weight of the whole bird. Indeed, in one Wood-
Pigeon they were equal to three thirteenths of the
total weight — a little less than a quarter.* Hence
the centre of gravity lies considerably below, though
not so very far below, the shoulder joints. But the
idea that it is good to have the centre of gravity of
a flying machine low down is altogether a miscon-
ception, a fact that can easily be put beyond all
doubt by experiments with small gliders. Weight,
placed low, tends to make the machine oscillate
and even swing right round. It is best that it
should be on a level with the supporting surface.
In the case of the bird, the chief supporting surfaces,
the wings, are always changing their relative position
as they are raised or lowered, and, of course, as they
move they to some extent raise or lower the centre
of gravity. This must make voluntary adjustments
for the purpose of recovering balance still more
necessary. But Professor Marey has pointed out
a curious fact which may make these oscillations
less difficult for the bird to deal with. When the
down-stroke takes place and the wings are lowered,
the centre of gravity occupies a lower position in
the bird, but the bird as a whole, except during
very rapid flight, rises. With the up-stroke, on the
other hand, there is a raising of the centre of gravity,
but a lowering of the bird ; hence, though the bird's

* Legal and Reichel, in the Jahresberichte der Schlesischen
Gesellschaft (1879), give ¥ .|^ ( = i^§ = more nearly | than A) as
the proportion of the total weight of the pigeon accounted for by
the three pairs of breast muscles; But this would seem to be a
mistake: Charadrius, according to them, comes next with f.

STABILITY, 25

body rises and falls, its centre of gravity travels
onward almost in a straight line.*

Points that make for Automatic Stability.
Though the position of the centre of gravity is
of no avail, the general build of the bird and the
elasticity of the feathers make for automatic stabil-
ity. To begin with, there is the question of curves. I
have already pointed out that a curved surface gives
more lift and less drift than a flat one. But anyone
who experiments with gliders soon finds out that
an excessive curve is fatal to stability. In the
preface to the Aero Manual (1910) it is stated that
the curve (the depth of the concavity) should not
be more than one-twelfth of the breadth. Imagine
a surface with a more considerable curve. The wind

W — — ^

Fig. 11.

Diagram to show the disadvantage of an excessive curve.

will impinge upon its upper side and the glider will
duck and descend rapidly to earth. But if the curve
is only slight, though the tendency to duck and dive
may, no doubt, arise, yet it tends to correct itself.
Imagine the glider launched on its way and develop-
ing a tendency to lower its head and raise its tail
unduly. In proportion as the tendency develops, in

* There is no need to discuss the question of pendulum stability.
It is possible to suspend a heavy weight from an aeroplane, and
there are theorists who hold that automatic stability may be thus
attained. It is obvious that in the case of a bird nothing of the
nature of a pendulum is possible. See Flight, Feb. 25, March 18,
and March 25, 1911.

26 THE FLIGHT OF BIRDS

proportion as the glider makes a smaller and smaller
angle with the horizon, the point at which the air
acts will be progressively nearer to the front margin,
and this will obviously tend to prevent a dive
downward.

Fig. 12
Diagrams showing the shifting of the centre of pressure.

This point requires some explanation. Take as
an illustration a sailing boat when it is tacking.
When it sails close to the wind, the air which strikes
against the forepart of the sail is deflected from its
course, and rushing towards the stern of the boat
forms a buffer which shields the rest of the sail from
the air, which would otherwise have impinged upon it.
The drawing of a boat tacking, on page 8, and
the accompanying diagram, will make this clear.
The principle is called the law of Avanzini. The
smaller the angle made by the wind with the sail,
the nearer the point of impact approaches to the
front edge. This is true no less of the slightly curved
surfaces of aeroplanes. As the angle of incline to
the horizontal is more and more reduced, the wind
acts at a point closer and closer to the front edge,
and thus the aeroplane may, possibly, correct the
dangerous tendency automatically, without the
aviator having to make readjustments.

To illustrate this principle I have made experi-
ments with a catapult which, working within
runners, threw pieces of cardboard horizontally,

STABILITY 27

the cardboard being set so as to have an upward
incline ; in fact as an aeroplane is set when it is
travelling horizontally. This was managed by
means of small wire carriers having various inclines,
on which the cardboard rested, and which were
themselves thrown with the cardboard. So far
from pitching head downwards, the cardboard
missiles would, even when the angle of deflection
from the horizon was small, rise in the air, and some-
times even turn over backwards, so strong was the
action of the air on the front margin.

In the bird's wing there is a further automatic
safeguard. It curves downwards at the back,
at any rate that part of it that is nearer to the body.
The wind acting over-strongly upon the downward-
curving back part of the wing might, if the whole
wing-surface were rigid, capsize the bird and send
him diving head-foremost downward. But the
elasticity of the feathers prevents an excessive lift.
They yield to pressure, and the reduction of their
curvature relieves the wing of any excess of pressure
on its hinder part.

The same principle holds good with regard to
lateral stability. If a strong gust or eddy of air
strikes the right wing while the left is struck with
less violence, the feathers of the right wing yield
to the rush of air and bend upward, so that the
very force of the gust to some extent reduces its
effect. Such elasticity would seem to be impossible
in the case of an aeroplane without dangerously
reducing its strength. And yet an aeroplane, from
its enormous breadth, is far more liable than a bird to
suffer from gusts falling with unequal force on its

28 THE FLIGHT OF BIRDS

two extremities. At present the aviator is at a
great disadvantage as compared with a bird. The
great expanse of his planes is in itself a danger,
and his machine has less power of automatic adjust-
ment. Moreover, the most experienced pilot is
a mere novice compared with a bird that has flown
many times every day since he left the nest. Still,
birds as flyers have already reached their zenith;
for aviators greater things are still possible.

Leaving the possibilities or impossibilities of the
future, I must return to the bird as he is, and call
attention to another point which promotes stability.
The extremities of the wings of the best flyers
have very little downward curve. If they are
travelling very fast through the air, the primary
wing-feathers may even bend slightly upwards,
and aviators have found that the upward incline
of their " planes " reduces the amount of rolling.
Thus inclined they give less support than if curved
downwards at the extremities, but the very fact
that the air is allowed to escape easily to right and
left is favourable to equilibrium. If an attempt
is made by bending the extremities downward to
check its escape, to coop it up, it may make a
sudden rush from the right or from the left con-
cavity and cause risk of a capsize. The wing,
amply concave as it is till it begins to taper, and
shallowing as it tapers, keeps in view the questions
of lift and equilibrium ; it holds the air, yet provides
for its escape. Sometimes when a bird wishes to
glide downward he will point his wings steeply
upward — a method often adopted by pigeons —
much more steeply than the right and left surfaces

STABILITY 29

of a monoplane. This attitude must tend to
ensure a comfortable equilibrium (PI. in, b).

The question of the escape of air from beneath
the curved surfaces seems to be of the utmost
importance. Mr. Pilcher's celebrated glider was
punctured with many small holes which were
intended to encourage a steady escape instead of a
sudden upsetting rush on one side. I cannot judge
whether this was a good plan for dealing with the
difficulty. At any rate, the wing of even a large bird
presents too small a surface to be treated in this
way. But it is probable that the notches between
the feathers at the wing's posterior margin tend to
prevent irregular escapes of air from below. The
great soaring birds, whose steadiness as they circle
with motionless wings is so marvellous, have the
great flight-feathers parted like outspread fingers.
Moreover, the force of the wind sometimes bends
them conspicuously upward, thus giving them an
incline that is recognized as favourable to equilibrium.

Voluntary Adjustments.

It cannot be gainsaid that there is much in the
bird's build that makes for automatic stability.
Nevertheless, since the wind, at low levels at any
rate, is a chartered libertine, full of capricious un-
expected eddies, such automatic adjustments are
altogether inadequate. The bird must be ready at
a moment's notice to give his mind to the question
of balance, and make conscious voluntary adjust-
ments. But even we ourselves find that movements
we are perpetually making tend, through force of
habit, to become automatic, and that in some cases

30 THE FLIGHT OF BIRDS

we are not conscious of any difference between the
voluntary and the involuntary. In the case of a
bird the line is still harder to draw, not only because
we cannot get within his mind, but because the
movements which we are bound to label voluntary,
since they cannot be mere unconscious reflexes, are
so largely instinctive. When the young Swallow
takes the first plunge from the parental nest and
trusts himself to the air, he finds at once that he
can fly ; the power of flight is instinctive. A child
has to learn by much practice to co-ordinate his
muscles ; no other young creature is so devoid of
instinctive skill. The young Swallow, though he
can co-ordinate his muscular activities well enough
to fly with some success, has, of course, much to
learn. At the outset he misses most of the flies and
gnats, and his parents have to come alongside and
put their captures in his mouth. But he seems by
instinct to spread his tail when it should be spread,
and, no doubt, though it is hard to see this, he takes
a harder stroke with one wing than the other, when
a harder stroke is required. For fore-and-aft balance
he depends largely on his tail. He has not a long
neck that he can bend or straighten out, and his legs
are so short and light that no movement of them can
have much effect. For lateral stability he must, as
far as voluntary adjustments are needed, depend
mainly on unequal wing-strokes.* That birds do
take unequal strokes has been clearly proved by the
camera in the case of Pigeons, Owls, Gannets and
some others (see PI. n). This inequality, sometimes
so conspicuous in a photograph, the human eye has

* See Chap. V.

PLATE II.

Right and left wings giving different strokes. A and B : Pigeons.
C : Young Gull ; the right wing only is flexed at the wrist. (See

Chap. II.)

To face p. 30.]

STABILITY 31

great difficulty in detecting, and but for the camera
we should have had to own that, though the bird
could hardly dispense with this method of balancing
and steering, we could get no absolutely conclu-
sive optical evidence. For fore-and-aft balance the
means employed vary very much according to the
build of the particular bird. Long-necked birds can
move their centre of gravity forward and backward
by extending or bending their necks. Ducks and
Geese usually carry theirs stretched out, Herons
habitually make a crook in theirs. Long necks
are usually correlated with long legs, and long legs
are no less serviceable in the matter of balance.
This is shown by the fact that the long-legged have
usually small, short tails, the long legs to some extent
taking the place of the tail as balancers. A Flamingo,
to take a conspicuous example, having legs of
enormous length and a neck to match, has no need
of much tail to regulate his fore-and-aft balance. A
very little extension or retraction of legs or neck
will set matters right if they are going wrong. But
not only do long legs make a great expanse of tail
unnecessary for balancing purposes, they must in-
evitably hamper its movements if it is to be pulled
downward with a view to checking speed or steering
to right or left. It may well be that this necessary
inefficiency of tail is in part the cause of the compara-
tively clumsy steering of these big, heavy-legged
birds.

It can be shown, too, that for purposes of balance
webbed feet very probably play a part of some
importance. Not only are they fairly heavy, but
they can be used as the tail is used, though not so

32

THE FLIGHT OF BIRDS

effectively. The following table brings out very
clearly the poverty of the long-legged and the web-
footed in point of tail. I give first the actual length
of tail, then the approximate weight of the bird.
In two cases I have had to take the weight from a
learned paper by two German ornithologists, and I
suspect that their hen Sparrow-Hawk was far from
plump. Lastly I have given the length of tail which
each of the birds has for one pound of its weight.
The results are very striking. Were I able to
give the area of the tails, they would be much

Bird.

Length
of tail.

Weight
of bird.

Length

of tail

for each

lb.
weight
of bird.

Notes.

Sparrow-Hawk (hen)
Sparro w-Hawk ( cock )

Ins.

8

6-5

5-3 oz.

(Legal &

Reichel)*

5-2 oz.

Ins.
24- 1

20

Tail very
broad.

Lapwing

41

6-7 oz.
(Legal &
Reichel)*

9-8

—

Wood-Pigeon

6

1 lb. 4-25 oz.

4-7

Tail very-
broad.

Common Wild-Duck

4-75

lib. 15-7 oz.

23

Tail

pointed,

small in

area, weak.

Common Heron

6-5

3 1b. 0-6 oz.

21

—

Curlew

462

2 lb 4 oz.

2-05

—

* Jahresbericht der Schlesischen Gesellschaft, 1879.

STABILITY 33

more so. But the area varies so much, according
as they are expanded or not, that I have found
it very difficult to give measurements. However,
we can bear in mind that the tail of the Sparrow-
Hawk, for example, is not only very long but very
broad, whereas the tail of the Duck is not only short,
but narrow and weak, its inefficiency being even
more marked than its small size.

It is noteworthy that the bird which stands
at the head of the list as being remarkable for size
of tail is not a particularly short-legged bird. Two
cock Sparrow-Hawks, of which I took measurements,
had each legs 7 inches long (the toes being included).
However, the legs are very thin, as if not intended
for standing, but, armed as they are with long,
efficient claws, for seizing a victim. It is often
maintained that Hawks hang their legs down during
flight, but this is certainly not usually the case.
However, they could on occasion be lowered, to give
the tail freer play, without much affecting the bird's
equilibrium.

The main function of the tail is to prevent loss
of equilibrium, and when large it plays its part
wonderfully well. In spite of its great expanse,
its weight is a negligible quantity and its working
is splendidly prompt. Evolution has done wonders
in thus metamorphosing the long, heavy tail of the
bird's reptilian ancestor. A number of vertebrae
have been compressed to form the pygostyle, the
small bony base of this wonderful piece of machinery.
What weight there is is almost all accounted for
by this little bone and the muscles that move the
large spread of feathers. The muscles have the

34 THE FLIGHT OF BIRDS

tail completely under command. At a moment's
notice they spread it, make it concave, lower it or
raise it, lower this side or that. If the tail is spread
and lowered, at once the hinder part of the body
is lifted (see Pis. in and ix). The opposite effect
will follow from the raising of the tail, so that it
may catch the wind but little. If a Lark be watched
through a field-glass as he rises facing a fair breeze,
he will be seen to be perpetually busy with the work
of correcting his fore-and-aft balance ; and it is to
his tail that he trusts. A Chaffinch, perched on a
rail with a fairly high wind blowing in his face,
keeps his tail perpetually at work. The tail is, in
fact, a fine balancer, even more important in this
capacity than it is as a rudder. Were it not for the
perfection of this balancer, a small bird could hardly
land without danger to his eyes in the chevaux de
frise of a furze bush.

There are other balancing movements still to be
considered. From our present point of view, the
way the wings are held is of great importance. I
have already pointed out that when a boat is
tacking (see fig. 4, p. 8) it is the front part of the
sail that does most of the work, and that similarly
when the gliding bird inclines his wings at a small
angle to the horizon, it is the front margin that
gives him most support. If, then, he holds his wings
fully expanded, so as to have as wide-stretching
a front as possible, not only will the lift be greater
but the centre of pressure (the point at which we
may consider the force of the wind as being, so to
speak, focussed) will move forward, and this will
tend to give the bird's body an upward incline.

STABILITY 35

And so, if he wishes to maintain his level or rise,
he must extend his wings to the utmost; if, on
the other hand, he wishes to descend rapidly, a
partial flexing is the measure to adopt. To the
aviator such a method is only to a very limited

Fig. 13.
Swift, gliding with wings fully expanded.

Fig. 14.
House-Martin, gliding with wings partly flexed.

extent possible ; he cannot half-flex his planes.
Nevertheless, he is able to slide down a steep incline,
much as a bird does. By means of his fore-and-
aft balancing apparatus, he sets his aeroplane
at the proper angle and shuts off steam. If he is

D 2

36 THE FLIGHT OF BIRDS

a skilled and experienced pilot, he is, apparently,
as steady as he rushes down towards the earth as
a bird could be. But, if he did lose his balance, it
would be fatal, whereas to the bird a momentary
loss of equilibrium is of no consequence.

Leaving aviators and their splendid achievements,
I must describe another attitude sometimes adopted
by a bird in gliding downward. He will extend his
wings to the full, but hold them slanting upward
(see PI. in). Obviously in this position the wings
give less support, and so he descends. But he
descends slowly, not with the rush that is character-
istic of the head-foremost downward glide. The
wings do not travel edgeways through the air and
so they check his pace. Their upward slant is, as
I have before remarked, advantageous to balance.

I must now conclude this brief investigation of
the bird's stability when on the wing. What we
see in the flight of birds — I am not now speaking
of soaring — is not a steady, careful maintenance
of equilibrium, but an instantaneous recovery of
balance whenever it is lost. The bird can afford
to be indifferent to the difficult problems which
this subject presents. He has something much
better than the power of maintaining equilibrium.
However the gusts and vagaries of the wind may
upset him, he can right himself at once. He owes
his wonderful stability to some extent to his fine
build and the elasticity of his feathers, but mainly
to manoeuvres and adjustments that cannot be
mere reflexes. The flying machine which he pilots
is admirably built : still it can never dispense with
a pilot. But his voluntary adjustments are largely

PLATE ill

A : Pigeon using tail for maintenance of equilibrium. (See Chap. II.
and Plates IV., V., VI. and IX.). B : Pigeon gliding. C : Gulls gliding :
(See Chap. II. and Chap. III.)

_To face p. 36.]

STABILITY 37

instinctive, and even the niceties of adjustment
that he has to learn must become through habit
almost automatic. And the result of it all — of his
fine build, his instinct, and his art — is the perfect
stability of the machine ; a thing to make the most
skilful aviator envious : for who ever heard of a
bird losing his balance and falling to the ground ?
There are, of course, trials to which he may prove
unequal. He may lose his way in a cloud, become
exhausted when crossing a wide stretch of sea, or
even fly stupidly into a telegraph wire. But an
accident from want of balance would be evidence
of morbid condition.

CHAPTER III.
MOTIVE POWER.

LEVERAGE — PROPULSION — PHASES OF THE WING-STROKE.

In describing gliding we have taken the motive
power for granted, assuming that the bird already
has momentum. We must now investigate his
method of lifting and propelling himself. The lifting
is the hard work, for the bird is so shaped that when
he sets himself at a suitable angle the air offers but
little resistance to his movement onward in a hori-
zontal direction. To lift himself he must put great
force into the downward beat of his wings, making
their extremities move with such velocity that the
air, the resistance of which increases as the square
of the velocity, will very soon offer effective support.
They will, in fact, become levers, each having its
fulcrum mainly near the extremity, the weight to
be lifted being, of course, the bird's body, or, more
correctly, the whole bird. The power is applied
quite close to the body (see fig. 21, Chap. vn). The
depressor muscle, the great muscle that springs from
the breastbone and covers it with its great expanse,
attaches to the humerus (or upper- arm bone) close
to the nearer end. With a lever like this there is
no economy of power — very far from it. What is
gained is rapidity of movement. A quick movement

MOTIVE POWER

39

of the near part of the wing will produce a movement
of astonishing rapidity towards the further end, near
the tips of the great primary- feathers.

If x (see the figure) moves half an inch, x will
move two inches. Preparatory to the down-stroke,
the wing is lifted till it points straight upward, its
anterior margin being turned in the direction of the
bird's flight. During the first part of its descent it

X

Diagram to show the velocity with which the extremity
of the wing moves.

cuts edgeways through the air. But soon it turns
face downwards and, the air opposing its descent,
it finds a fulcrum. But, of course, there is some give.
The fulcrum that the air supplies to the wing is, like
the fulcrum that the oar finds in water, an imperfect
one. When, however, the bird is taking full-length
strokes, the wings appear to move with a far longer
sweep than is really the case. With each stroke the

40 THE FLIGHT OF BIRDS

body rises, and this means a relative lowering of
the wings.

It is a wonderful thing that the air can supply a
tolerably firm support, something that will do duty
as a fixed point. Archimedes undertook to lift
anything if he could find a fulcrum for his lever.
The bird finds a fulcrum where Archimedes would
not have thought it worth while to look for one.
The bird when flying is, in fact, taking a number
of jumps. Often when he appears to be travelling
in a horizontal line he is really, as Professor Marey
has shown in his wonderful photographs that give
successive phases of the process, rising and sinking
with each down-stroke and up-stroke. Thus the
bird's apparently horizontal line of progress is often
an undulating one. But when he is travelling with
great velocity, then there is no drop between the
strokes ; of this Professor Marey has obtained
evidence. But there is probably some reduction
of pace. Even though there is no rise with each
stroke, the bird is nevertheless taking a series of
jumps. And the marvel of these jumps, with no
better take-off than the air, no amount of thinking
can do away with. A man who is accounted a good
high jumper can do very little if he has a poor take-
off — if the ground is spongy. We all find it very
hard work walking over soft snow when at each step
we sink up to our knees before we find anything
firm and resistant beneath our feet. We walk
slowly and with labour along a beach where the
small pebbles let our feet sink in. We climb with
effort up a volcanic cone where, each step that we
take, the small rounded ashes let us slide downward

PLATE IV

Phases of the wing-stroke : photographs of Pigeons. A : Ready for the
down-stroke. The series is continued on Plates V. and VI. (See
Chap. III.)

To face p. 40. ]

MOTIVE POWER 41

til] we lose almost all that we have gained. But the
bird has to deal with a material that seems far more
shifty and ^independable than small pebbles or
volcanic ashes, or than snow at its worst. He over-
comes the difficulty by means of levers calculated
to give the utmost rapidity of movement. The
muscular effort required is great, but his muscles
are strong, and it is long before they tire.

Propulsion. — Phases of the Wing-stroke.

But the bird has not only to lift himself, or to
maintain the altitude he has already gained. He
must also have onward momentum. Were this
wanting, he could not even lift himself, for air has
little or no supporting power when it has just
been disturbed. He must, therefore, be perpetually
advancing to fresh columns of air that have not yet
been shattered by the beating of his wings. I have
already pointed out that, when birds are flying in
flocks, each takes care to keep clear of the backwash
of the bird in front of him — takes care to avoid tracts
of air that have already been disturbed ; that a
Pigeon, when he has a string tied to his leg, cannot
maintain himself in air, however wildly he may ply
his wings, when he has reached the end of his tether ;
that when a Kestrel hovers without advancing there
is always a breeze, so that each wing-beat descends
on fresh, unbattered air. For ordinary flight the
wings must be so adjusted as to propel as well as
lift. This the bird can effect only if the front part
of the wing is lower than the hinder part. Thus the
parallelogram of forces comes once more to his aid.

Let f b (fig. 16) represent a section through the

42 THE FLIGHT OF BIRDS

bird's wings, f being the front and b the back margin.
The resistant air (w in the figure) will be equivalent
to a wind blowing vertically upward. It will act
at right angles to the plane of the wing, and the line
representing its action will point not only upward
but forward. During horizontal flight the front
edge of the wing is slightly at a lower level than the
back. But when the bird is rising and taking very

Fig. 16.

Diagram showing the effect of the lowering of the front margin
of the wing.

energetic strokes, then the required incline is not
obtained by that method only. The wing moves
forward as it descends, so that at the end of the
down-stroke, instead of making a right angle with
the body, it points more forward than outward.
When the wing is in this position, there is the down-
ward slope that is wanted, from its base at the
shoulder to the tip. One method passes gradually
into the other ; indeed, the wing is inevitably shoved
forward when the air lifts its hinder margin. On

PLATE V.

B

1

£>

Phases of the wing-stroke, continued from Plate IV. A : Last phase
of the down-stroke. B : Beginning of the upstroke. C : Feathers
bent upward during the up-stroke {see Marey, Vol des Oiseaux,
p. 268, and Plate at end). D : Up-stroke continued. Continuation
of series on Plate VI. {See Chap. III.)

To face p. 42.]

MOTIVE POWER

43

the forward-downward movement as giving the
required incline I wish to lay stress, because treatises
on flight do not, as far as I know, recognize this way
of obtaining the downward slope, but speak only of
the depressing of the front margin relatively to the
back (see Pis. iv and v).

Fig. 17.

Gulls flying (after Marey). A. — 25 photographs per second.
B. — 50 per second.

I am now going to describe more in detail the
different phases of the stroke. For this I must
depend to some extent on Professor Marey's photo-
graphs. Other photographers, myself among them,
taking snapshots, have caught the various wing
positions. He shows us the various phases in series :
they follow one another at the rate of 25 or of 50
per second (see fig. 17 and Pis. iv, v, vi, vn).

44 THE FLIGHT OF BIRDS

I will first give some account of a long and com-
plete stroke. Preparatory to the down-stroke, the
wing is raised till it points vertically upward, its front
margin being turned in the direction of the bird's
flight. There may then be a moment's pause, the
wing, as it were, resting before it strikes its blow. In
the case of gulls the next move seems to be a slight
bend at the wrist- joint. After this begins the serious
work. The wing descends with lightning speed, so
fast indeed that this early phase of the down- stroke
does not always appear in the series when photo-
graphy has succeeded in depicting all the other
phases. The great rapidity at this stage seems to
indicate that it is not till the wing is approaching the
horizontal that it begins to feel the resistance of the
air and do its work of lifting and propelling. When
this work is going on, the upward bending of the
primary- feathers leaves us in no doubt about the
fact. As the wing descends it points more and more
forward. The way in which this is brought about
is highly interesting. The big muscle which lowers
the wing attaches to the front part of the lower face
of the humerus (upper-arm bone) (see. Chap, vn,
fig. 21). Its pull, therefore, tends to lower the front
of the wing relatively to the hinder part by rotating
the bone. But the air, acting on the feathers that
spread out rearward, greatly aids the muscle, lifts
the hinder part of the wing, and encourages the rota-
tion. But the work of the air does not end here.
As soon as the wing has an upward incline from front
to back, it cannot but move forward ; the mere
action of the air on a surface so inclined cannot but
bring this about. Thus the upward incline from

PLATE VI.

Phases of the wing-stroke, con t inure I from Plate V. A : Nearly the
same as Plate V., and from a dinVrenl point of view. B, C, D : Up-
stroke continued. (See Chap. III.)

[To face p. 45.

MOTIVE POWER 45

front to back, without which the bird would not
make headway, is obtained first by the raising of
the back part of the wing relatively to the front,
and later, as the stroke advances, by the forward
movement of the descending wing, which brings it
about that the extremity occupies a position lower
than and in advance of the base. For a moment
let us consider the working of the wings in combi-
nation. With the body they form a kind of funnel
— obviously one side of the funnel is missing, but
this is unimportant. Caught in this funnel and
deflected from the wing- surfaces, the air impinges
upon the body and lifts it. When the wing has
strained forward and downward till it can strain
no further, the muscles at length relax. The wing
is no longer rigidly extended, but slightly bent at the
elbow-joint, and soon at the wrist also. If the bird
is rising and has little onward momentum, the
Elevator muscle does the work of lifting. The great
flight- feathers, which during the down-stroke have
been pressed close against one another and so have
made the wing impervious to air, are now slightly
rotated, so that interspaces are left which allow the
air to pass, and thus the raising is effected without
much opposition (see Chap, vn, fig. 24). If, on
the other hand, the bird has much way on, the air
itself effects the lifting and little work on the part of
the muscles is required. After the strain of the down-
stroke, the Depressor muscle ceases its contraction,
and, perhaps, the Elevator gets to work. In any
case the front margin of the wing is no longer de-
pressed relatively to the hinder margin, but is lifted.
The wing lets the wind have its way, and is carried

46 THE FLIGHT OF BIRDS

back, almost unresisting, but occasionally, as photo-
graphs show, it resists enough to cause a backward
bending of the flight- feathers. It looks as if the
bird, when his wings are being lifted by the rush of
air into position for a fresh stroke, checked them for
a fraction of a second in order to save himself from
loss of altitude. It is possible this check in the
course of the up -stroke may be the normal thing,
or it may be only occasional (see PI. v). When
the wing is thus raised by the rush of air — more
strictly by the resistance of the air to its momentum
— the flight- feathers are pressed against one another
and there are no gaps to make the lifting easy. But
no such help is required. The wing is blown back
as far as it has freedom to go, and at the end of its
rearward movement it is no longer facing as it was ;
its under- surface is facing outward and its anterior
margin is looking towards the bird's head. When
the wing is in this position it is easy to raise it com-
pletely and bring it forward ; in fact it moves edge-
ways. Anatomy supplies very remarkable evidence
that the raising of the wing requires little effort.
Whereas the muscle which lowers the wing is red,
ridged, and granulated, the Elevator muscle is paler
and exposes a smooth surface when it is cut. The
former is by far the better class of muscle, capable
of long, unflagging effort.

In ordinary horizontal flight most birds take a
much shorter stroke than the one I have just
described, nor is the wing pointed much forward
(see PI. vn ). The upward incline from front to
back that is needed is obtained by a slight lowering
of the front margin relatively to the back, mainly

PLATE VII.

Gulls flying with short stroke. A : Lesser Black-backed Gulls.

B : Black-headed Gulls ; the lower bird on the left with wings

bent at the wrist, shows an early phase of the up stroke. (See

Chap. III.)

MOTIVE POWER 47

towards the extremity of the wing, and this gives
propulsion enough. This short stroke is the one
adopted by gulls in their ordinary leisurely flight.
The wings give the air a sharp slap, and this with a
bird so well built for flight and so skilled is very
effective. It is to be noticed that, when gulls are
taking short, leisurely strokes, the wings during the
down- strokes are very distinctly curved from their
base to their extremity. This curve prevents a too
easy slipping away of the air and so increases the
wing's lifting power ; but when a stronger stroke
is taken the primary- feathers bend upward.

The question how a bird lifts and propels himself
I have now briefly answered. In other chapters I
shall try to describe the build of this living flying-
machine, to my thinking the noblest of all craft that
sail the air (see Chaps, vn, vm, and xi).

CHAPTER IV.
STARTING.

THE PRELIMINARY JUMP LOSS OF ALTITUDE BETWEEN WING-
STROKES — THE WING'S FREEDOM TO ROTATE AEROPLANES

MUSCLES BIG BIRDS AND SMALL.

The Preliminary Jump.

I have shown how a bird when flying maintains
and propels himself, when we have given him in
imagination a good start. We have now to study
his method of starting.

Let us suppose that he is standing on the ground ;
if he is to fly he must somehow get clear of it. For
this purpose his long, strong legs are of great service
to him ; it is owing to them that he is so good a
starter. Such is the bird that I think of as typical.
Not only are his legs strong, but they are long,
unless we compare them with those of a Wader or
a Heron, birds that for their particular mode of
life have developed an inordinate length of leg.
The Puffin and the Swift, by their helplessness on
level ground, call attention to the remarkable leg-
power of the ordinary small bird, or bird of moderate
size. The Swift and the Puffin cannot take the
preliminary jump with which flight must begin.
Bulky, short-legged birds all rise with difficulty ;
the jump that should in a moment obtain for them
freedom of wing action is beyond their power.

PLATE VIII.

A and B : Pigeons starting to fly. B : Avery hurried start. C : Pigeon
rising with body inclined steeply upward. D : Herring-Gulls starting ;
photographed in September, when they were moulting. (See Chap. TV.)

STARTING 49

It is not that they are deficient in lifting power,
for it is said that an Eagle can carry a weight as
great as its own, and I have seen a Falcon flying
off with a victim that was not far short of itself
in point of size.

Whether big or small, birds all fold their wings
neatly upon their backs, where they cannot possibly
interfere with freedom of leg action. In order
to appreciate the excellence of this arrangement,
we must compare the bird with the bat or with the
pterodactyle, whose wings were remarkably bat-like.
Extending, as they did, far back and attaching to
his legs, they must have been as bad an encumbrance
as long skirts. Though, no doubt, a fine flyer in
a bat-like style when once launched on his way,
the pterodactyle was a poor starter. The bird's
legs, on the contrary, are not sacrificed to his wings
(see PI. viii). Unless he is one of the specially
bad starters, he jumps lightly into the air and is off.

With regard to the heavy, lumbering way in which
many big birds rise, a great deal is to be learnt
by watching a Heron start to fly from level ground.
Unlike the Condor, the Comorant, the Puffin, and
the Swift, he has no difficulty in getting under
way. True, he does not rise with a steep incline
as an ordinary small bird or a Pigeon can, but a
gradual ascent he carries out without difficulty.
He drops from the top of his long legs on to the
spacious fields of air and is at once clear of the earth,
with ample room for the plying of his wings. On
the other hand, the big short-legged birds and
small birds like the Swift, with very short, feeble
legs, have no room for a full sweep of their wings,

50

THE FLIGHT OF BIRDS

unless they have, by some means other than a
jump, attained some elevation. Hence the difficulty
they have in starting from level ground. But even
when this full sweep is possible big birds are still
only capable of ascending with a gentle incline, and
this is a fact that we must try to account for.

Loss of Altitude between Wing-Strokes.

To my thinking the explanation is this : since
the big bird necessarily takes a longer stroke and
requires more time to raise his wings for a fresh
downward beat, he must inevitably lose more
altitude between wing-strokes than the small bird.
Professor Marey gives the rate of stroke for a
number of birds as registered by means of scientific
apparatus.*

Strokes per

second

Sparrow

.. 13

Duck

9

Pigeon

8

Marsh-Harrier

.. 5|

Screech-Owl . .

5

Buzzard

. . 3

In the case of a Sparrow it is evident that there
is no time for a drop between the strokes, though
compared with those of most insects even a
Sparrow's wings move slowly and heavily. The
leisurely working of a Heron's wings is familiar
to everyone who ever takes the trouble to observe
birds ; usually he flaps along with only 130 strokes
a minute or even slightly less. The Marsh-Harrier

* See his Vol des Oiseaux, p. 100 k

STARTING 51

has a much quicker beat, but even with such a rate
as his a drop between the strokes is quite possible
unless he has plenty of way on. And now we are
getting to the explanation of the big bird's method
of rising. In order to avoid losing altitude between
the strokes, he must take care that he has momentum,
and if he is to have momentum he must be content
to ascend by a gentle incline.

The Wing's Freedom to Rotate.

Moreover, there is a want of freedom about his
wing-movements which makes him incapable of
anything but a very gradual ascent. If he were to
incline his body steeply upward, after the manner of
a small woodland bird that, making for a gap in the
dense spreading boughs overhead, mounts almost
vertically, he would have to rotate his wings in a
way that is impossible for him ; he would have to
lower the front margin relatively to the back, or
else they would beat in such a way as to drive him
backward instead of lifting him. In fact, he has
too little freedom at the shoulder. He cannot
set his wings as a steep ascent requires. The
small bird's wings, on the other hand, rotate so
freely that even when he sets his body with a steep
upward slant he can still turn them over so that
they have an up-and-down beat and raise him sky-
ward.

But though, speaking generally, the small bird is
capable of a steeper ascent than the big bird, yet it
would be a great mistake to imagine that if we were
to arrange birds according to their weights, from the

K 2

52 THE FLIGHT OF BIRDS

lightest to the heaviest, we should at the same time
be arranging them according to their angles of ascent
— the angle which each makes with the horizon when
the line of his ascent is as near to the vertical as his
build and his powers allow. In the case of all very
big, bulky birds, I believe the wing rotates reluctantly
and with difficulty at the shoulder. The Gannet,
Pelican, Cormorant, Eagle I have tested, and found
that they have very little power of lowering the front
edge of the wing relatively to the back ; one Eagle
was a partial exception. I have never had a live or
a freshly-killed Condor at my disposal, but there is
reason to believe that the Condor is among the
stiffest of the stiff. The way in which Condors are
trapped in Chile — it is described by Darwin in his
Journal of Researches (Chap, ix) — supplies indirect
evidence of the great bird's limitations. The plan
is to place a carcass "ona level piece of ground
within an enclosure of sticks with an opening, and,
when the Condors are gorged, to gallop up on horse-
back to the entrance, and thus enclose them : for
when this bird has not space to run it cannot give
its body sufficient momentum to rise from the
ground." In fact a Condor cannot rotate his wings
and set them as they must be set if his line of
ascent is to make a large angle with the horizon.
As far as I know, all birds of great bulk have this
defect, but when we come to birds of medium size we
find great variations, and it soon becomes apparent
that it is largely a question of habitat and environ-
ment. The Pheasant and Duck — I have tested the
Mallard, the Sheld-duck and the Teal as representa-
tive of the Ducks — have great freedom of movement

STARTING 53

at the shoulder, and can, moreover, point their wings
forward so that at the finish of the down-stroke they
have an upward incline from tip to base. On the
other hand, the Partridge and the Herring-Gull have
very little power of rotation. The striking contrast
between the Partridge and the Pheasant throws a
great deal of light upon the question. The Pheasant
is a denizen of woods and has often to make for an
opening in the branches that shows itself almost
directly over his head. Heading straight for it, he
points his body almost vertically upward, but, in spite
of that, his wings have an up-and-down beat, and turn
the concavity of their nether side towards the ground
and their upper convex surface towards the sky.
The Partridge, on the other hand, frequents plains
and open fields where there are no entanglements
to make a nearly vertical ascent even an occasional
necessity. A Wild Duck sometimes finds it expe-
dient to mount upward from her nest among the
bushes in the same style as the Pheasant just
described, and I have seen a Wigeon, without any
apparent advantage to himself, shoot up thus from a
large piece of open water. All birds, if we except
those that frequent only open water, bare cliffs, bare
hills or unwooded plains, may find, any moment, that
they have to make a rapid ascent up a steep incline ;
life itself may depend upon it. Hence great freedom
at the shoulder is very common. I have observed
it not only in the Pheasant and the Duck, but in the
Jackdaw, Crow, Raven, Chough, Jay, Magpie, and
Quail. No wing, I think, rotates more freely than
that of the domestic Pigeon. Were it not for this,
the bird in PL viii could not achieve, as he is

54 THE FLIGHT OF BIRDS

evidently doing, the feat of an almost vertical
ascent.

The big and bulky birds then, such as the Gannet
and the Condor, having a much slower wing-stroke
than the light-weights, must make it their first
object to attain momentum, otherwise they will
lose altitude between the strokes. When they
have got way on, they can rise, but the line of their
ascent is a gentle slope. For a steeper ascent the
set of their wings unfits them. But there are birds
of medium size, such as the Duck and the Pheasant,
which, striking with very great rapidity and rotating
their wings as freely as any small bird, are capable
of raising themselves almost vertically through
the air.

Aeroplanes.

In a country like England, with trees and hedges
almost everywhere, an aeroplane capable of a
steep ascent is a great desideratum. Captain
Brooke- Popham, writing on military aviation, says
that " with no wind a fully-loaded machine, with
observer, could get off a hard level field in a length
of 120 yards and clear a fair hunting hedge at the
end. Our Air Battalion " Farman " can do this
in 90 to 100 yards without any difficulty."* Thus
even the " Farman " requires a great deal more
space than a Condor or a Comorant, birds noted
as slow, lumbering starters, and though aeroplanes
are constantly being improved it may well be
doubted whether a steep ascent will ever be
achieved.

* The Army Review, January, 1912, p. 89.

STARTING 55

Muscles.
To rise in air before he has got up pace is hard
work for any bird unless he can get the wind to
help him, for a great strain is put upon the muscle
that lifts the wing. And here I may call attention
to the remarkable development of the Elevator
muscle in the Pheasant. Its weight amounts to
nearly one-third of that of the Depressor. It is
very pale and has little lasting power, but for a
brief effort it is very effective. Its development
in the Duck is considerable, but not equal to what
is found in the Pheasant. As soon as the bird
begins to travel rapidly, there comes an easier time
for the overtaxed Elevator, for the resistance of
the air to the onward impetus is sufficient to lift
the wings, and the impetus is due mainly to the
work of the great Depressor muscle. Hence the
Depressor not only lowers the wing, but indirectly
lifts it. The Elevator is not fitted for long-sustained
effort. The Depressor is a redder, rougher, more
granulated muscle, and its different colour and
texture are indicative of superior quality.*

Big Birds and Small.

As a rule, when a small bird flies, his line of
flight is undulating. For him to rise is easy ; a
few strong, rapid strokes lift him. He then partly
flexes his wings and glides onward and slightly
downward. This is very noticeable in the case of
the Woodpecker. He flexes his wings more than
most birds, and so the dipping character of his flight

* For more on this subject, see p. 45 and Chap, vii : Muscles.

56 THE FLIGHT OF BIRDS

is more conspicuous. The big bird, on the other
hand, dare not lose altitude in so reckless a way,
since when he has once lost it he with difficulty
recovers it. He ploughs steadily on, whereas the
small bird with a few rapid strokes gains altitude,
then, like a bicyclist utilising his free wheel, glides
restfully and rapidly onward, not minding the loss
of some of the height he had gained. His stroke
is more rapid, but he is able to take frequent
easies.*

I have now compared and contrasted big birds
and small, but even the biggest birds that fly are
not very big ; as compared with the larger mammals
they are diminutive. Why, among all the birds
that fly, are there none that weigh even half as
much as, for example, a Zebra ?

Helmholtz, dealing with this subject, produced a
formula which, backed as it was by the authority of
a great man, was too readily accepted, regardless of
the fact that it was not founded on data obtained
by experiment. There is nothing so misleading
as mathematics when the premises are unsound.
Helmholtz started with undeniable facts. If a bird's
linear dimensions be multiplied by 4, then the area
is multiplied by 16 (4 2 ), and the bulk, which must
nearly correspond to the weight, by 64 (4 3 ). So far
good. He showed that the weight increased more
rapidly than the supporting area. But when he
went on to maintain that the power required to lift
the bird increased at a still more rapid rate — that, in
the case I have taken, it would be 128 (4i) — then he
was building a theory without a proper foundation

* On big birds and small birds see Chap, i, pp. 18-22.

STARTING 57

of proved fact. Even now, though the subject has
been much studied, it would be very rash to venture
on a formula. The big bird has great advantages.
He can manage, as we have seen, with a relatively
smaller expanse of wing, for the area, being greater
absolutely, does not so readily allow the escape of
air at the margins. Moreover, his wing, being longer
absolutely, is a more powerful lever. The great
weights which Eagles, for example, carry show that
there is no deficiency of lifting power. But the big
bird's wings have not the easiness of rotation at the
shoulder- joint which makes it possible for a Green-
finch, for instance, or a Pheasant, to rise with a steep
incline ; he cannot put himself in the right attitude.
It does not appear, however, that bulk in itself is
any handicap.

If this be so, it may well be asked why even big
birds are quite small when compared with the larger
mammals. I have already pointed out that the big
bird, if his legs are short, has difficulty in beginning
a flight, and so lacks a very important accomplish-
ment. Then why are they not all mounted on stilts,
like the Flamingo ? But legs of such length, or even
the half of it, would for many birds be most incon-
venient appendages, for a diving bird most of all.
They would not help him to rise from the water, and
they would be clumsy things beneath the surface.
And thus among big birds there are many that for
purposes of flight are handicapped by shortness
of leg. Diminutive size brings with it another
advantage. The small bird comes more rapidly to
maturity. In his second spring, when he is not yet
twelve months old, the Blackbird has already paired

58 THE FLIGHT OF BIRDS

and is busy nesting. The Gannet is known, I
believe, not to nest till his fifth year ; the Eagle
comes to maturity later, possibly not till his tenth
year. In rate of reproduction, therefore, a most
important matter for the species, the small bird has
a great advantage. And thus we can account for
the small size of birds compared with that of mam-
mals without recourse to any such theory as that of
Helmholtz. The small bird is a better starter than
the big, and he comes to maturity more quickly.

CHAPTER V.
STEERING.

A VARIETY OF METHODS GOOD STEERERS AND BAD.

A Variety of Methods.

A bird can steer when his tail is gone. A Rook,
when some accident has robbed him of this useful
rudder and balancer, can still make shift. He is
not like a ship left rudderless. It is evident,
therefore, that the tail is not the bird's sole steering
apparatus. If he wishes to steer to the left his
usual method is to fling himself on his left side,
the left wing pointing downward and the right
wing upward, the two being in line with one another,
while his head is pointed in the direction in which
he wishes to travel (see PL ix). Then he can no
longer progress along his former line of advance,
for the expanse of his wings will check him. He
travels, therefore, to the left, i.e. towards the point
towards which his head is directed. But how does
he effect the necessary change of balance ? There
is no doubt that he occasionally gives a harder stroke
with one wing than the other, a thing which the
camera sometimes detects, though it is difficult
for the eye to see it clearly. On Plate n are some
photographs in which the wings have been caught

60 THE FLIGHT OF BIRDS

at a moment when they were not held symmetrically.
Another plan is to bend sideways at the waist,
so as to move the centre of gravity towards one
side. I was long doubtful whether this plan was
actually adopted. At length I have obtained a
photograph of a Gull, taken from below, where the
waist, thus bent, is clearly shown, but unfortunately
this photograph will not stand reproduction. Below
I give some measurements which show that birds
that are good steerers have greater suppleness of
waist than clumsy steerers.

The tail also is undoubtedly used for steering,
though, I believe, it is more frequently of service
in maintaining or restoring equilibrium. The rule
is, as I have shown, that if the legs are long the
tail is small, and as it is indisputable that long
legs are very useful for balancing and of but little
use in steering, we may infer that the tail is more
a balancer than a rudder. Nevertheless, it is of
much use in steering. Though its edges look to
right and left, it can be made an effective rudder
by the lowering of one side more than the other.

Web-footed birds probably use their feet occa-
sionally for steering purposes, but I doubt if their
action counts for very much. When a Duck, for
example, is hurtling through the air, if he lowers
one foot, the resistance of the air will double it up,
not expand it. It is very different when a Duck
in swimming kicks backward ; the water acts
upon the under-surface and spreads the webs to
their fullest extent. Some birds when alighting
use their feet in the same way. A Gannet, for in-
stance, kicks hard in order to correct his balance

PLATE IX.

A and B : Gulls steering by throwing themselves on one side. A :

Steering to the right and looking to the left. C and D : Pigeons

showing the tail used as a rudder. (See Chap. V.)

To fare p. 60.

STEERING 61

and get into the right attitude for settling. It is
a clumsy performance.

The movement of the head to one side or the
other has, no doubt, some slight influence on the
balance. But the skull is very light, a great part
of it as thin as paper, and one may occasionally
see a Gull look to the left while he steers to the right
(PI. ix ), just as a skater, who is quite at home
on his skates, can make such minor movements
without in any way upsetting his balance. This
subject naturally brings to mind the wonderful
way in which Mr. Cody showed the lateral stability
of his biplane. He carried a passenger who stood
at a distance of ten and a half feet from the centre
where he himself was piloting. When a bird
turns his head, it is far more probable that he is
directing his eyes, to right or left, towards some
object that has attracted his attention, than that
he is regulating his balance. Mr. Bentley Beetham
has a capital photograph* in which he has caught
five Gulls in the act of turning their heads to the
right. This may possibly be due, as he suggests,
to a local current of air with which they all equally
have to cope. But it seems far more likely that
they have all suddenly caught sight of some object
of interest, a fish or shoal of fish, at no great
distance. What would one not give for a photograph
of a large flock of birds scudding through the air
high aloft — for a photograph taken at the moment
when by some common impulse, as if following some
leader, they each and all change their course ? Might
we not see, if we could obtain such a photograph,

* British Birds, Dec, 1910.

62 THE FLIGHT OF BIRDS

all the heads turned one way, the object being to
follow with the eye the movements of the leader or
leaders of the flock ?

Good Steerers and Bad.

There are good steerers and bad steerers.
Contrast the Swallow, the Lapwing, or the Sparrow-
Hawk, those adepts at sudden swerving and
doubling, with the Duck ! The Duck becomes the
slave of his own ponderous momentum, and changes
his course slowly and with effort. The Swallow,
the Lapwing and the Sparrow-Hawk have all of
them a fine expanse of tail, while the Duck's is small
in area and, what there is of it, feeble. Efficiency
or inefficiency of tail, no doubt, accounts for a
great deal. But it is not the only factor to be
considered. The Duck, though very strong, is
lacking in agility ; the three other birds have far
more suppleness, I believe, notably at one im-
portant point. Though birds' backbones are,
below the neck, very stiff, yet they allow of a good
deal of bend, either up and down or sideways, at the
waist. I have measured the amount of sideways
bend in some few species and found that, as far as
my far from complete evidence goes, good steerers

Angle formed by backbone
bending sideways at waist.

Kestrel 141 5°

Swallow 150°

Swift 153-5°

Common Tern . . . . 155°

Kestrel (another specimen) . . 156°

Domestic Duck . . . . 165°

STEERING 63

have supple waists and poor steerers comparatively
stiff ones, the Duck being a good deal the stiffest
of those I have examined.

It is difficult to account for the difference between
the two Kestrels, but there is no doubt that the
Duck is, of the birds in question, decidedly the
stiffest. In fact these measurements make it a priori
probable that suppleness of waist counts for
a good deal. The result of the sideways bend
must be that the bird is thrown on to its side — the
attitude which is assumed when a sudden turn is
to be made.

For steering purposes, then, a bird has various
methods at his disposal. He can take unequal
wing-strokes or bend at the waist. Either of these
means will put him in the attitude in which we
see him steering to right or left, one wing pointing
downward, the other upward. And he can use
his tail as a rudder. Some birds may, no doubt,
steer partly by means of their feet, but I doubt
whether a foot is a very effective rudder. The
Duck's webbed feet do not seem to make him an
adept at turning.

CHAPTER VI.

STOPPING AND ALIGHTING.

A bird alights without any jar. Let us imagine
that when we first catch sight of him he is flying at
some height above the ground. Wishing to descend,
he will give his wings an upward slope and float down
in the style usually preferred by Pigeons, or more
probably he will slant his body, from tail to head,
downwards and, partly flexing his wings, glide rapidly
towards the earth. On nearing his landing place he
will suddenly let his hind- quarters sink, and give
his body an upward incline ; his wings, spread wide,
present their whole under-surface to the air. This
soon checks his momentum. But if he wants to
make a very sudden stop he gives a stroke with his
wings, and this, when the body is nearly upright,
as a glance at one of the photographs {see PI. x)
that illustrate the process will show, must bring him
at once to a standstill. And just after landing,
perhaps (notably if he is a Tern, a Lapwing or a
Pigeon) he will raise his wings high above his head
as if to stretch and refresh the muscles, in which
attitude he looks very beautiful. A beautiful thing,
too, is the folding of the wings ; it is all so quickly
and so neatly done. A Tern or even a Pigeon when
he alights has all the grace of the " Herald Mercury,
new lighted on a heaven-kissing hill."

PLATE X.

A and B : Pigeons alighting. Though in B the feet are
still clear of the window-ledge, I think the bird is giving
his wings the final stretch that with many species is pre-
liminary to the folding of them. C : Pigeon checking speed
before alighting. (See Chap. VI.)

To face p. 64. J

STOPPING AND ALIGHTING 65

As birds are so careful in alighting to avoid all jar,
it is remarkable that some species have habits which
would seem likely to cause concussion of the brain.
It is astonishing that the Nuthatch, for example,
when he has been half the day hammering at nuts
with tremendous vigour, yet suffers no bad conse-
quences. But we must bear in mind that the bird's
neck, with its peculiar saddle-and-rider vertebrae,
is more supple than a snake, whereas the backbone,
except just at the waist, is remarkable for its rigidity.
Hence, probably, the care and dexterity in alighting
that contrast so strikingly with the reckless use of
the beak and head as a hammer by the Nuthatch,
the Woodpecker, and other birds. It must be owned
that there are some birds which, when they are
alighting, are very unlike the "Herald Mercury."
Among these we must count the Gannet. When he
gets near his nest upon some cliff, he paddles hard
with his legs, and at last settles clumsily down. Mr.
Bentley Beetham (British Birds, May, 1911) has
some very good illustrations of the Gannet 's style
of alighting. One of them shows a bird that has so
lost command of his movements, that he is flopping
most ungracefully onto his nest. His breast is
resting on the pile of dry seaweed, his outspread
wings on the rock at either side. It is an astonishing
attitude, but it must be borne in mind that the
Gannet is in the habit of taking headers from a great
height, and that his breast is shielded by air cavities
beneath the skin, first-rate air-cushions that greatly
reduce the shock when he dashes into the water.
This may account for his descending breast foremost
onto his nest. But the Cormorant, who has no

66 THE FLIGHT OF BIRDS

air-cushions, though he is the Gannet's near relative,
is also clumsy in alighting, and Mr. H. F. Witherby
has described to me how Cormorants, wishing to
alight upon a post, will sometimes make a bad shot,
pass it, and have to turn and try again. A Gull will
alight gracefully enough on the water, but when he
is aiming at a particular spot on a rock, for example,
he will often paddle as awkwardly as a Gannet. The
Lapwing is in a very different category ; he is noted
for his power of suddenly stopping, of making every
possible turn. When you get near his nest he has a
way of flying at you in threatening style, then
suddenly checking himself and making off. He is a
perfect master of bluff. A pair of Lapwings, by such
menacing swoops and turns, will drive off a Crow
whom they suspect of looking for their nest. In
order to stop suddenly, what is wanted is a great
expanse of feather, and I cannot help associating
the Lapwing's remarkable wings — they are so
strikingly broad at the extremity — with his well-
known shock tactics.

I have already remarked on the skill which small
birds show in alighting on their perch. Were a
Linnet not very clever at making all the required
adjustments, she would seldom reach her nest, some-
where deep in a gorse bush, without accident.

CHAPTER VII.
THE MACHINERY OF FLIGHT.

THE BREASTBONE AND THE CONNECTED BONES MUSCLES AND

QUALITY OF MUSCLE THE SCAFFOLDING OF THE WING

PNEUMATIC BONES STDTFNESS OF WING EXPANSE OF BONE

THE SPREADING OF THE WING STRUCTURE OF A FLIGHT-
FEATHER — MOULTING — LEGS.

When we carve a Partridge or a Grouse, since other
questions seem at the time of more pressing interest,
we seldom give a thought to the fact that we are
slicing and disjointing what has been a most mar-
vellous flying machine. Yet so it is. When we
carve the breast, we first cut through the Great
Pectoral, the powerful muscle that lowers the wing.
Lying below it we find, easily distinguishable, a
much smaller, lighter-coloured muscle — in the
Grouse and its kin markedly lighter. This smaller
muscle lies in the angle between the keel and the
sternum (or breastbone) proper ; its work is the
lifting of the wing.

The Breastbone and the Connected Bones.

The framework, the bony skeleton, is wonderfully
adapted to the purpose to which it is put. It is
most important, to begin with, that there should
be a wide expanse of bone from which the great
flight muscles may spring. And so the area of the
sternum is increased by the large projecting keel.

F 2

68 THE FLIGHT OF BIRDS.

The Ostrich has no keel, and is incapable of flight.
The Hoatzin, that curious South American bird,
according to all observers a most feeble flyer, has
the front part of the keel missing. In every case
the shape of the keel has its significance and is well
worth observing, since it indicates the style of flight.
The Duck and the Guillemot, having very long
breastbones and keels, have of course very long
flight muscles and a long muscle is capable of more

Fig. 18.

Breastbones of (1) Guillemot — actual length of keel 5k inches.

(2) Falcon. Drawn to scale. Compare that of the Adjutant,

a Stork famed for his soaring.

contraction than a short one. A glance at their
breastbones, therefore, tells us that they are birds
that fly with a long stroke. A Guillemot's wings are
so small that a long, strong stroke is a necessity
for him. A Duck, during horizontal flight, does
not raise his wings very high, but the stroke is pro-
longed till the wings can strain no further downward.

THE MACHINERY OF FLIGHT

69

The Tern and the Falcon, on the other hand, have
short, deep keels, and therefore short but big and
strong flight muscles. Accordingly, their ordinary
stroke is short but powerful. It is highly important
also that there should be a firm pivot on which the
wing may rest as it moves. During part of the
wing's down-stroke there must be considerable

Breastbone and connected bones of Adjutant — actual length of
keel 4| inches. CI. : Clavicle ; Co. : Coracoid ; Sc. : Scapula.

pressure inward, and were the skeleton to give,
were it to supply only an unsteady wobbling pivot,
vigorous flight would be out of the question. How
is the pivot formed ? There is a strong bone, the
coracoid, which springs from the front part of the
breastbone and points forward and outward. It

70 THE FLIGHT OF BIRDS

slopes more outward in strong than in weak flyers,
and, as a rule, more in big birds than in small. At
the shoulder -joint the coracoid is met by two other
bones, the clavicle or merrythought (which varies
much in make and strength), and the scapula or
shoulder-blade, and the three bones together form a
first-rate pivot, so that the strong wing-beats do not
shatter the bird's framework.

2U

Fig. 20.

Clavicle of (1) Tern ; (2) Eagle. Drawn to scale.

Muscles and Quality of Muscle.

From the keel and from the outer part of the
sternum (thus covering up the small Elevator
muscle), and also from the clavicle, springs the Great
Pectoral — the Depressor muscle ; it attaches by a
short tendon to the front part of the under-surface
of the humerus (upper-arm bone), and, attaching
where it does, its pull tends to lower the front part
of the wing relatively to the back. And springing
as it does not only from the breastbone but also from
the clavicle, it pulls the wing, not downward and
backward, but simply downward. The underlying
Elevator sends out a long tendon that passes

THE MACHINERY OF FLIGHT

71

through a little tunnel formed by the three bones,
which meet at the shoulder -joint, and after passing
through the tunnel attaches to the upper side of the
humerus, near to its preaxial or front edge. Its
action is to erect the wing upon its pivot. Attaching

Fig. 21.

Humerus of right wing of Eagle. Actual length 6f inches. D :

Flat area near the front margin of the under-surface, where the

tendon from the Depressor muscle attaches.

where it does, it must necessarily bring the wing
into the proper position for beginning the next stroke
with its front edge looking in the direction of flight.

72 THE FLIGHT OF BIRDS

Sometimes, in order to carry out a movement, two
muscles have to antagonise one another. When a
bird wishes to check his speed suddenly, he lets his
body hang downward, sometimes almost vertically,
and holds his wings so that their under-surface faces
in the direction of his flight. Now, if the Depressor
were alone holding the wing, it would lower the front
relatively to the back, and the resistance of the
air would reinforce its action, making the wing turn
its under-surface downward as in ordinary flight,
whereas for checking speed it must face to the front.
To orient it thus, by holding fast the front margin
of the upper face of the humerus, is the work of
the Elevator. The antagonising of one muscle by
another is, of course, constantly going on. Were
it not for this, with its steadying effect, no movement
could be carried out with precision, no attitude could
be maintained. There are, of course, other muscles
which contribute to the working of the bird's wing,
and, notably, there is a third pectoral, the office of
which is to draw the wing back. But it is no part
of this short treatise to describe the work of minor
muscles.

I have already alluded to the different qualities
of muscle found in birds. Muscle of the highest
quality — capable of an enormous number of very
rapid contractions in rapid succession — consists of
fibres in which the fine striation in the direction of
the pull of the muscle is much obscured by irregular,
highly granulated cross ridges. In colour it is a rich
red. The fibres consist of small, contractible
fibrillse, and are buried in a granular material known
as sarcoplasm. Muscles which are capable of long-

THE MACHINERY OF FLIGHT 73

continued exertion are rich in sarcoplasm, which
would seem somehow to supply food to the muscle
proper, to the contractile fibrillse ; at any rate this
is a reasonable theory that explains the facts.*
Now the muscle which in strong-flying birds is
reddest and, in section, most ridged and granulated,
is the Great Pectoral that lowers the wing. This
is a muscle of great size, as I have already pointed
out, and its output of unflagging energy is truly
astonishing. The Elevator muscle is paler and
smoother, and almost certainly of far inferior quality.
Much less is demanded of it. As soon as the bird
has got up pace, the resistance of the air lifts the
wing, and the Elevator has little to do. In fact the
great Depressor may claim to lift the wing, since to
it is due the velocity which relieves the Elevator of
its work. In the Blackcock, the Red Grouse and
the Ptarmigan, the paleness of the Elevator forms
a striking contrast against the rich red of the great
muscle under which it lies. Among birds whose
flight muscles I have examined, the Guillemot is
the only one whose Elevator is as rich a red and
almost as stringy as the Depressor. Is this explained
by the fact that the bird uses his wings in swimming ?
Has he to raise his wings in the water by muscular
effort ? And is this due to the fact that when he
swims he has less pace than when he flies ? In the
Chicken, since it seldom uses its wings, all the
Pectoral muscles are very pale and smooth. The
wild Jungle Fowl, from which our domestic bird
springs, uses its legs far more than its wings, and, I
believe, domestication has not much altered the

* See Starling's Physiology on quality of muscle (pp. 87, 88).

74 THE FLIGHT OF BIRDS

character of the different muscles. Remarkable also
is the large size of the Chicken's Elevator muscle.
Its wild kinsman, haunting the jungle as it does, has
to rise to its perch without any wind to help it, so
that all the work of raising the wing must fall upon
the muscle. But the effort required is not a pro-
longed one, and so the muscle is pale. In the
Sparrow-Hawk and, I believe, in other birds of prey,
the Elevator is very small. Since they usually start
to fly from a perch at some height above the ground,
there is no difficulty in getting up speed enough to
make the office of the Elevator largely a sinecure.

The brown flesh on a Chicken's leg, though the
human palate pronounces it inferior, is, nevertheless,
when regarded as muscle, of distinctly better quality.
The Fowl is primarily a runner, not a flyer ; hence,
presumably, the darker colour and the greater
strength and endurance of the leg muscles. More-
over, these muscles are kept on the strain throughout
the night; by the bending of the knee and ankle
joints the machinery of muscles and tendons through-
out the leg is set to work and the bird's toes strongly
grip the perch. His weight keeps the legs bent, and
the bending of the legs keeps muscles and tendons
to their work. Were they to relax for a moment the
bird would probably lose his life ; he would fall to the
ground and some hungry carnivore, that happened
to be prowling about, would seize him. But though
in the Fowl the leg muscles are darker in colour than
those of the breast and wings, they are not in section
ridged and granulated like the great Pectoral of
birds that are strong flyers. In the Moorhen, I have
noted that the leg muscles are rather more ridged

THE MACHINERY OF FLIGHT 75

than is the Elevator, but not so much as the De-
pressor. In most birds that I have examined, I have
found that the leg muscles show less ridging and
granulation even than the Elevator. But the
Moorhen makes great use of his legs. He is both a
swimmer and a runner. When not swimming he is
generally walking on the grass beside his pond or
stream, busied with the search for worms.

Before I leave the subject of quality of muscle I
must point out that the inferiority of the pale to the
red has been noticed also in thoroughbred horses.
Mr. J. B. Robertson, in his very interesting paper
on the Principles of Heredity applied to the Racehorse
(p. 22), writes : "The pale fibres greatly predominate
in the tissues of a pure sprinter, and the red fibres
in those of a stayer."

The big muscles, the work of which I have
described, are all massed upon the sternum. Even
the muscles which bend or straighten the wing at
the elbow spring not from the humerus or upper-
arm bone, but from the top of the coracoid and the
anterior end of the shoulder-blade respectively. I
once cut off the wing of a domestic Pigeon as close
as possible to the body and found that it scaled just
under £ oz. The bird weighed 13i oz. Thus the
two wings together accounted for just under one-
eighth of the whole. It is wonderful that such
strength can be combined with such lightness. And
not only is the wing, as a whole, light ; what weight
it has belongs almost entirely to the bones and muscles
of the near part. This Pigeon's wing balanced,
when rested on a wire 2J inches from its base
and 10 inches from its tip. The great primary

76 THE FLIGHT OF BIRDS

feathers (i.e. those which arise from the hand) point
mainly outward and only slightly backward. Hence
the length of a bird's wing is due largely to feathers,
and they are proverbially light things. Their
strength is no less remarkable than their lightness.

The Scaffolding of the Wing. Pneumatic Bones.

The scaffolding of the wing is itself very light.
The thickest of the bones, the humerus, is hollow in
big birds that are strong on the wing. In some the
bones are hollow right on to the finger-tips ; there
is an opening in each bone at the near end (see fig.
22) ; a thin pulmonary membrane enters there, and
thus they are filled with air that has passed through
the lungs. The Gannet is a good example of this
complete aeration. Many small birds, however,
though first-rate flyers, have all their wing-bones
solid ; the Swallow, for example. The Swift has
only the humerus pneumatic. The differences be-
tween nearly related birds are remarkable. The
Gannet's remarkable pneumaticity I have already
mentioned ; his near relative, the Cormorant, has
only the humerus pneumatic. Such examples seem
to make it clear that aeration is an adaptation to
the life and habits of the particular species, not an
unvarying character firmly established in certain
orders of birds.*

The fact that large birds have more aeration than
small demands explanation. But the explanation

* Besides the wing-bones, the Gannet has the following bones
pneumatic : The vertebrae, the greater part of the sternum
(not the keel), the ribs, the coracoid, the ischium and the femur.
There are also ample air-cushions beneath the skin that covers
the breast.

THE MACHINERY OF FLIGHT 77

is not far to seek. The small bird would gain but
little in lightness by the aeration of his bones, since
each bone consists almost entirely of its exterior
shell. The big bird, with his stout, bulky bones,
will gain far more. Here is a case in which we may
appropriately quote some geometrical facts. We have
seen above that if we take two cubes, the side of one
of which is twice that of the other, then a face of the
larger one is four times the area of a face of the
smaller one, and the cubic content of the larger is
eight times that of the smaller. Thus an increase
in the length and girth of a bone means a far greater
increase of the space within the outer shell. Obvi-
ously, then, a big bird stands to gain more in lightness
by the hollowing of the bones. How he manages
to do with so very little marrow — the bones having
only a very thin lining — is another question to which
I hope to return (see Chap. xi).

How much the bigness of the bone has to do with
pneumaticity is made clearer by the facts which
follow. If we make measurements of the humerus
of a Skua, or other Gull — the Gulls have very little
aeration — and of an Eagle as the representative of
the birds which have a great deal, we find that the
girth of the Eagle's bone is disproportionate to the
bird's superiority in length of wing. And the
explanation, no doubt, is this ; the Eagle requires
much greater strength in his wing -bones than does
the Gull. Even a small increase in length of wing
means a considerable increase in the pace at which
the extremity will move. And, as we have seen, the
resistance of the air increases as the square of the
velocity. It is easy to see, then, that the Eagle and

78

THE FLIGHT OF BIRDS

other big, long-winged birds have wing-bones of
larger girth (of a girth, that is, disproportionate to
their superiority in length of wing) in order that
they may be able to bear the far greater strain put
upon thern. A proportionate increase in weight
would, no doubt, have caused difficulty, and this
has been obviated by the aeration of the bones.

Fig. 22.

Humerus of (1) Skua; (2) Rhinoceros Hornbill ; (3) Sea Eagle

— actual length 6f inches. Drawn to scale. F : Foramen, where

the bronchial membrane enters, on the upper surface.

To bring out this point more clearly I have taken
the humerus of the Skua as the standard, and have
calculated what would have been the length of the
same bone in the Sea Eagle if it had been built on
the same lines.

After all this I have to admit that there remains
a very puzzling case on which I cannot throw any
light. The Hornbills are slow, heavy flyers, they
are not very big, and they are the most pneumatic

THE MACHINERY OF FLIGHT

79

Girth of
humerus.

Humerus.

Aggregate length
of wing-bones.

Actual
length.

Length
propor-
tionate to
girth.

Actual
length.

Length
propor-
tionate to
girth of
humerus.

Ins.

Ins.

Ins.

Ins.

Ins.

Skua

II

H

*i

13|

13f

Sea Eagle

1A

6|

7 7

'20

20 T V

22^

of all birds. The huge hollow beak is characteristic
of the whole skeleton (see fig. 22).

Stiffness of Wing. Expanse of Bone.

It is highly important that the bird's wing when
extended should be rigid — its bony framework, that
is. It has elasticity where it is wanted, the elas-
ticity of the great feathers, which tends, as I have
shown, to make the bird automatically stable during
flight. The stiffness of the scaffolding of the bird's
extended wing is most remarkable when we compare
it with the human arm. If we extend one of our
arms horizontally to its full length with the palm
of the hand downward, we can, while still keeping
the upper arm lifted, bend the elbow- joint and
point the fore-arm vertically upward. As the
bird's wing descends with great velocity through
the air, it does not give at the elbow, but remains
rigid. But the stiffness of the bird's wrist is still
more striking ; when the wing is extended hardly
any up-and-down movement at that point can

80

THE FLIGHT OF BIRDS

take place. The rigidity there is more easily
brought about owing to the fact that of the two
small rows of wrist-bones the further one has been
fused with the hand-bones. When the fore-arm
is brought into line with the upper arm, the hand
also falls almost into line and is held tight in that
position by ligaments. As soon as the wing is
flexed at the elbow, it bends also at the wrist —
bends backward, that is, towards where digit 5

MC

Skeleton of wing of Adjutant (from the elbow). D 1, 2, 3 :
Digits 1, 2, 3. MC 1, 2, 3 : Metacarpals 1, 2, 3. R : Radius.
RC : Radial carpal. Ul : Ulna. UC : Ulnar carpal. The
marks left by the little pockets in which the secondary feathers
are planted can be seen on the ulna.

would be if it existed — and then at the latter joint
much freer upward movement also is possible. And
so the wing is rigid when rigidity is wanted, and
pliant when pliancy is in demand {see fig. 23).

THE MACHINERY OF FLIGHT 81

It is essential, too, that there should be a large
expanse of bone, a firm foothold for the great flight-
feathers. The lizard, or lizard-like reptile, from which
birds are undoubtedly descended, had a very wobbly
hand. Even the Archaeopteryx, the most ancient
of birds, though it was beyond doubt a true bird and
not a bird-like lizard, had three long fingers which
show no sign of becoming fused together. With
birds as we know them, things are very different.
The three surviving metacarpals, or hand-bones,
enlarged by the fusing with them of the further
row of carpals or wrist -bones, are themselves fused
together, the two of the three that are the most
important fused both at the near and further ends.
Of the three surviving digits two are fused ; the
little thumb, that supports the bastard- wing, remains
independent and insignificant ; and the result of
all the fusing is that a broad, firm platform is
provided for the feathers. Thus we see that the
scaffolding of the wing when extended is remarkably
rigid in spite of its lightness ; its wide expanse of
feathers is remarkably elastic, and is planted on a
broad, firm base.

The Spreading of the Wing.

The machinery by which the wing is spread is well
worth study. The rapidity with which it does its
work is wonderful, and the accomplished result is
most beautiful. When the triceps muscle straightens
the wing at the elbow- joint, it is straightened also
at the wrist, not absolutely but nearly ; there are
special muscles for giving the finishing touch. Of
the two bones in the fore-arm the front one (the

82 THE FLIGHT OF BIRDS

radius) is pulled towards the body when the elbow
is straightened and this straightens the wrist. A
further result inevitably accompanies the first
straightening movement. There is an elastic liga-
ment that stretches from the armpit to the extremity
of the digits. Through this pass all the secondary
flight-feathers and, in every bird in which I have
traced its course, all the primaries (those that spring
from the hand). Part of this ligament is shown in

Fig. 24.
Wing viewed from below (after Alix, with a slight alteration,
see text). AM : Muscle and tendon supporting anterior mem-
brane, Dl : Digit 1. El: Elastic ligament. F: Flexor carpi
ulnaris, dividing into two. T : Tendons connecting muscle with

quills.

figure 24. But it is easy to see the whole of it in
any bird's wing when the small covering feathers
have been removed. The straightening of the wing
stretches the ligament and the great flight-feathers
spread like a fan.* {See PL xi.)

* There are differences in different families. In some the liga-
ment, instead of being pierced by the quills, fastens only to their
under-side.

THE MACHINERY OF FLIGHT 83

There is another ligament which fastens to the
under-side of the quills of the secondaries nearer to
their base. Moreover, the secondaries grow back-
ward, and plant themselves each in a little pocket
of very tough fibrous tissue. These pockets are
firmly rooted in the hinder of the two bones of the
fore-arm (the ulna) and leave, each of them, their
mark on it (see fig. 23). Planted thus, the second-
aries, though very firmly gripped, have freedom to
rotate. The primaries, pointing outward as they do,
run along above the hand and finger bones and are
held with a still stronger grip.

It is marvellous machinery which spreads the
wings so rapidly, and which, having all its heaviest
parts massed upon the body or very near to it,
leaves them splendidly light, especially near their
extremities. But we have not yet come to the end
of the contrivances. During the down-stroke the
secondaries press each against the one above it and
so prevent the passage of air, whereas during the
upstroke they let it pass. They are held fast by a
number of stays ; there are the little pockets just
described, the ligaments and a sheet of fibrous
tissue extending from their bases to the great liga-
ment. There are also little tendons that connect
the secondaries with a muscle which, arising from
the further end of the humerus, attaches its other
extremity to one of the wrist-bones (carpals) and to
one of the metacarpals, or hand-bones, beyond (see
fig. 24). Working unopposed it bends the wrist.
When the wing is straightened it helps by its opposi-
tion to hold the wrist- joint tight, thus strengthening
a point where the strain of flight is much felt. When

84 THE FLIGHT OF BIRDS

the wing is bent the muscle lies in a slightly curved
form — we notice it when we are getting the meat
from between the two long bones of a chicken's
wing. It is straightened out when the wing
straightens, and this, combined with the outward
slope assumed by the feathers as they spread,
stretches the little tendons that arise from it and
are fastened to the feathers. The tendons slope
outward, away from the shoulder, and attach to
the under-side of the quills, or rather to the
fibrous tissue with which the quills are surrounded.
M. Alix (in his Appareil locomoteur des Oiseaux) makes
some of them at any rate attach to the further edge
of the lower face of the quills, and gives them the
credit for rotating the feathers during the down-
stroke, so that they press against one another and
make the wing impervious to air. But when I have
scraped these little tendons away in the wing of a
freshly-killed Pigeon I have found that, when I
extended the wing, the feathers still took their
proper position. The extending of the wing causes a
stretching of all the important stays — the ligaments,
the sheet of fibrous tissue, and these tendons — and
the ligaments may claim the largest share of the
credit for the spreading and marshalling of the
feathers. During the up-stroke the secondaries
are firmly tethered, but are no longer marshalled
for action. There are interspaces as there are
between the leaves of a folded fan that has seen
much service. The feathers being now no longer
pressed tight together, the air can pass between
them. When, however, the bird is flying fast and
the wings are lifted, not so much by muscles

PLATE XI.

A : Wing of Lark, showing the elastic ligament that holds the
primary and secondary feathers. Another ligament nearer to
the base is faintly shown. B : Primary wing -feathers of Black
Vulture (on left) and Pink-footed Goose, upper surface ; of
Heron, under surface ; and outer tail-feather of Heron. In all
the inner web is the broader. (See Chap. VII.)

[To face p. 85.

THE MACHINERY OF FLIGHT 85

as by the resistance of the air, the feathers during
the up -stroke are pressed against one another and
the passage of the air is prevented. But this causes
no trouble. The wing is very rapidly swept back-
ward and upward ; it turns its front edge in the
direction of flight, and when it is oriented thus it is
easily straightened and moved forward into position
for beginning the next down-stroke.

During the down-stroke the air helps the living
machinery in its work, making it still more effective.
The outer webs of the feathers (see PI. xi) are very
narrow compared with the inner ones, and the result
is that the air acts much more strongly upon the
latter during the down-stroke and also, when there
is much momentum, during the up-stroke, with the
result that each feather is rotated and has the inner
side of its vane pressed very closely against the one
that lies next to it and above it on the side nearer
to the body.

Though on the surfaces of the wing nothing is
visible but feathers, yet for no small amount of the
expanse two membranes can claim the credit. What
is called the anterior membrane stretches from the
head of the clavicle (or merrythought) to the hand
(see fig. 24). In the Gannet it is of great breadth
and is so hung that it not only increases the area of
the wing, but sloping steeply downward, as it does,
to form the front margin, it deepens the wing's
concavity and makes it, near to the body, what a
parachute should be. In most birds the membrane
is slung with only a gentle slope from its front edge
backward. In all it is at once stretched when the
wing is spread. The other membrane lies farther

86 THE FLIGHT OF BIRDS

back, in the armpit, and fastens the wing to the
bird's side.

It is worth while noticing that though muscles
springing from the body in the main control the
movements of the whole wing, yet there is a good
deal of local independence. There are niceties of
adjustment which depend on local muscles. Though
the triceps extends the upper arm, the fore-arm
and the hand, yet the two united fingers, and with
them the great feathers they carry, are not under
its sway but depend upon special muscles to spread
them to the full and carry out minor movements.
The little bastard-wing has also its own muscles —
more muscles than one would expect to be at the
service of so insignificant a piece of machinery.

Structure of a Flight-Feather.
For the bird flight without feathers is obviously
an impossibility. If the scales of the bird's reptilian
ancestors had remained mere scales, the ancestors
would have remained reptiles still, condemned to
crawl the earth. The scale has been glorified till
it is hardly recognisable. First comes the quill with
the dried remains of the pulp — the pulp that was
there when it was alive and growing — still visible
within it. Above is the rachis, or shaft, grooved
down its front face (see Pis. xi and xn). From
the shaft spring the barbs sloping towards the tip of
the feather, from the barbs branch out the barbules
or radii. From those of the barbules that are on
the far side of the barb (the side farther from the
base of the feather) spring the barbicels (diminutive
of a diminutive !) that fasten barbule to barbule and

PLATE XII.

(From Life and Evolution.)

The barbs, barbules and barbicels of a flight-feather. A and
C x 90 diameters ; B x 250. A : A barb. C : A pair of barbs
showing interlocking. B : A pair of barbules, showing the barbi-
cels on the one nearer to the tip of the feather.

To face p 86.

THE MACHINERY OF FLIGHT 87

thus give the feather its elasticity, besides making it
impervious to air. Compare for a moment a great
flight-feather with one of those whose business is
merely to clothe the bird and retain its animal heat
— a poor, weak, fluffy thing. The barbicels have
another name, hamuli or hooklets, from their
shape. On the side of the barb which is nearer to
the base of the feather, the barbules have no bar-
bicels, but their endings resemble hairs, and these
hair-like endings are neatly folded together so as to
form a kind of hem. The opposite set of barbules
lie over these smooth-ending ones, and among them
they insert their hooklets. The hooklets slide along
the barbules at moments of strain and stress, and to
this, very largely, must be due the elasticity of the
feather. The number of barbules is enormous ;
according to Dr. Gadow, over a million in one large
flight-feather.

Here is another very interesting point: on the
margin of the barbules opposite to that which is
armed with barbicels are rough knobs. These are,
I believe, the vestiges of barbicels which, being
useless, are in process of disappearing. The feather
is in fact thrice pinnate. The main shaft branches
into barbs, the barbs into barbules, the barbules
formerly branched, I believe, into barbicels on either
side. But the barbules on the side nearer to the base
of the feather have altogether lost their barbicels
and have now only hair-like endings, while the other
set have only one member of each pair properly
developed, the other member being represented only
by a mere vestige, so that there are only rough knobs
to match the barbicels.

88 THE FLIGHT OF BIRDS

From whatever point of view we look at it, a flight-
feather is a wonderful thing. It provides a large
expanse to support the bird's weight, it is elastic,
it is light and at the same time strong. Moreover,
it is renewed every year, and in such a way that the
bird does not even for a day lose his power of flight.

Moulting.

A living machine differs from a man-made machine
in many ways, and notably in this, that it has as an
indispensable characteristic, the power of self -repair.
When a bird's feathers are broken or worn out, they
must somehow be replaced. Now nearly all birds
that fly shed their flight-feathers gradually and in
pairs, so that, though during the moult they are not
at their best, yet they can always rise on the wing
(see PI. vni, 4). The Goose has somehow earned
a widespread reputation for stupidity, but the most
stupid thing that he does is not, I believe, generally
known. He moults so rapidly that for a time he
is reduced to helplessness. In the island of Kolguev,
the Samoyeds drive thousands of moulting geese,
who can only swim or run, into great nets and thus
provide themselves with a store of food for the Arctic
winter.

But, after aJl, this rapid method of moulting would
involve but little danger for the Goose if man had
not arrived on the scene, since his way of life became
stereotyped. Many birds, not geese alone, have
failed to find any means of escape from their enemy
with his ever-changing method of attack. There
are other birds which shed all the primary-feathers
simultaneously without any disastrous consequences,

THE MACHINERY OF FLIGHT 89

e.g. the Ducks. Mr. J. L. Bonhote has made a
special study of the subject, and, quoting from him,
I am able to give some very interesting and instruc-
tive facts. The simultaneous moulting of the
primaries occurs among the Divers, the Guillemots,
the Razorbills, the Puffins, the Auks, and besides
these among the Rails, e.g. the Moor-hens, the Land-
rails, and the Crakes. Nearly all of them are water
birds, and in the water they are fairly safe from
their ordinary enemies. Some of them can dive,
others can hide among reeds or under bushes ; the
Geese by force of numbers can, probably, beat off
a bird of prey. A sea bird, if he is to moult in this
fashion, must be a true diver in habit and in plumage,
if not in name, and thus we find that the Cormorant
(who is always wanting to dry his wings on a rock)
and the Gannet moult gradually. When we turn to
land birds the case of the Corncrake does not surprise
us, for though he is a migrant, yet in his day-to-day
life he trusts more to his legs than to his wings.
The case of the Coot is perplexing ; unlike the Moor-
hen, he is capable of flight throughout his moult ;
but, as Mr. Bonhote points out, he is the only member
of the Rail family that frequents estuaries and open
sheets of water, and he is, in addition, an indifferent
diver.*

Legs.

I have already pointed out how some birds use
their legs in balancing and, possibly, for steering.
Indirectly the great leg-power possessed by most
birds is a most important aid to flight. In order to

* See " Eclipse Plumage and FHghtlessness " by J. L. Bonhote,
in the Field, March 24, 1906.

90 THE FLIGHT OF BIRDS

rise with ease from the ground, they must be able
first to jump into the air. Most birds have an active
springing gait, and are good jumpers, their legs being
built much in the same way as those of a horse or
an antelope ; the ankle-joint is raised high above
the ground, and they walk or run upon their toes.
The springiness is combined with remarkable light-
ness. The bird's foot is made almost entirely of
skin, bone, and tendon ; not like the human foot,
fleshy and full of nerves. It is worked by long
tendons attached to muscles that spring from the
top of the leg- bone (the tibio -tarsus) or even from the
base of the thigh-bone, and, running in grooves under
the ankle-joint, give to the foot its springiness and
to the toes their wonderful grip. The fusion of the
two rows of ankle-bones with their bigger neighbours
makes the ankle-joint an excellent pulley. Alto-
gether the bird's wings have in the legs very able
assistants. Comparatively few resemble the Swift
in having very short and feeble legs ; but, as I have
pointed out above, there are a considerable number,
mostly big, bulky birds, which, owing to their
shortness of leg and small power of jumping, have
difficulty in starting to fly from level ground.

CHAPTER VIII.
VARIETIES OF WING AND OF FLIGHT.

CURVE — NARROW AND BROAD WINGS STYLES OF FLIGHT

FLIGHT IN FLOCKS THE WHIR OF WINGS.

Curve. Narrow and Broad Wings.

I shall first consider the question of curve, or, as
the aviators call it, of camber — a question of the
utmost importance. The wings of all birds are a
good deal curved from front to back, so that as they
descend they catch the air in a concavity and have
plenty of lifting power. In some birds the front-to-
back curve extends throughout the whole length of
the wing, though in all there is some shallowing
towards the tip. The Jay and the Red-legged
Partridge (and their allies) supply good examples of
a curve that shallows near the tip of the wing but
does not disappear. In a Jay, of whose wings I made
measurements, the depth of the curve — measured
just on the near side of the starting-point of the
bastard-wing — was 1 inch (see PI. xv and also xiii
and xiv ). The breadth of the wing at this point
was 5 1 inches. Thus the curve is 1 in 5|, a curve
such as no aviator would ever think of using. But
it must be remembered that during the down-stroke
it is much reduced by the pliancy of the feathers,
which yield to the pressure of the air. The span

92 THE FLIGHT OF BIRDS

from wing-tip to wing-tip was only 18 inches. In
fact it was a very short, broad, much-curved wing.
The Pheasant's wing belongs to the same class ; it
is very short (in a hen bird of which I took measure-
ments, only 20 J inches) ; very broad (6 inches) ;
and the curve in this particular bird was no less than
1J inches — If in 6 ! (see PI. xiii).

The Hoopoe's wing is very much of the same build
— unfortunately I cannot give exact measurements
— and yet, when the season of migration comes
round, many Hoopoes manage somehow to flap
across the Mediterranean. As a rule migrating birds
are characterized by wings of a very different make,
and it sometimes happens that the wing is the
feature in which they present the most striking
contrast to their non-migratory kin. The clamorous
Reed- Warbler, that is resident in Egypt, shows by
his wings that he does not migrate — they are so very
short. The migrant Reed- Warblers have decidedly
longer and more pointed wings. As to the Hoopoe
and its short, broad wings, it must be remembered
that though it manages to cross the Mediterranean
when the conditions are favourable, yet its migration
flights are comparatively short. Some, I believe,
do not migrate at all. If they are delayed by clouds
while crossing the Mediterranean, their strength is
apt to give out, whereas Herons and Swallows, to
take two examples, have a reserve of vitality and
will fly round a steamboat for hours, till, perhaps,
the weather clears again, when with unflagging
strength they will continue their journey.

To pass on to the Red-legged Partridge, I found
the depth of the curve to be 1 inch, as in the Jay, but

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VARIETIES OF WING AND OF FLIGHT 93

the width of the wing was only 4| inches, whereas
the Jay's was 5f inches wide. The wing-tip to wing-
tip measurements were 18| inches, hardly an advance
upon the Jay. It is a very short wing, and though
narrower than the Jay's, it is a broad wing if we
compare it with those of most birds. The Moor-hen,
also, has a rounded and not very long wing, with a
good deal of front-to-back curve, shallowing, however,
very much near the extremity. The measurements
in one specimen were : curve, T 7 ^- inch ; breadth,
4 £ inches ; wing-tip to wing-tip, 21 J inches. The
breadth is very little greater than in the case of the
Red-legged Partridge ; in length the Moor-hen has
decidedly the advantage, and the gaps between the
extremities of the feathers are much less ; in other
words, the wing is more finished and more efficient.
Here in England we look upon the Moor-hen as a
stay-at-home bird, but it will cross the Alps in
search of a warmer climate if need be. Frozen-out
Moor -hens from northern or central Europe often
accomplish this feat.

In other birds we find the curve much more reduced
towards the extremity of the wing ; in some it dis-
appears almost entirely. In the Thrush the last
inch and a half is nearly flat. In the Starling
(PL xv ) there are quite 3 inches with very little
curve. To take a bird of larger build, the Curlew's
wing (PL xiii) shows a great reduction of curve
as soon as it begins to taper to a point ; the last
4 inches are nearly flat. Moreover the curve has
not the same character throughout. For a distance
of some 9 inches from the body, a front margin, nearly
an inch broad, is flat ; behind that the downward

94 THE FLIGHT OF BIRDS

curve begins. Further out the whole breadth of
wing is curved ; for some distance from the front
margin there is an upward slope, then a downward
slope sets in. In the nearer region, also, there is not
a simple curve maintained throughout, but the
feathers in one part are more bent down than in
another, so that the back edge presents a curiously
undulating line. The Sparrow-Hawk's wing has
similar curves, but they are much less pronounced.
What the exact significance of all this complication
may be, presenting so marked a contrast to the
uniform curves of an aeroplane, it is difficult to say.
Probably there is some significance, since in birds
that are strong on the wing the whole mechanism
of flight down to minute details is so efficient.

Sometimes we may see the wing-curves in their
full beauty in a live bird, but it is only for a moment,
and one wishes to look at them quietly and study
them. The thing is, if one has a freshly-killed
specimen, to cut off the wings close to the body and
pin them out back downwards at their full stretch.
Treated thus the wing retains its curves and a great
deal of its beauty. If it is pinned face downwards
and flattened, it conveys by no means so good an
idea of what it was during the life of the bird. I
have tried by photographs to show the outlines of
various types of wing, the depth of their concavities,
and the undulating curves seen in, for instance, the
Curlew's wings. Some points come out distinctly,
others the camera fails to see or does not see clearly.

To show the gradual tapering and flattening of
the farther half, perhaps no wings are better than
those of the Gannet and the Tern (PI. xiv), and

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VARIETIES OF WING AND OF FLIGHT 95

they are things of wonderful beauty. We are
probably right in drawing from this flattening the
inference that the swifter movement of this part of
the wing makes a deep curve undesirable. The near
part moves more slowly and is more parachute-like
in character. The farther part, besides the momen-
tum of the bird as a whole, has the great rapidity
of stroke which sends it with a sudden dash both
forward and downward. We can hardly doubt that
the short, rounded wing is the primitive one. The
wing of Archseopteryx, most ancient of known
birds, did not taper to a point. And the clumsy
flyers — those with rounded wings have a compara-
tively feeble flight — must have preceded the skilled
flyers. The Hoatzin, with his very short, rounded
wing, cuts a very poor figure in the air. The Hoopoe
and the Jay are not strong flyers. It is not only
that their wings are short and very broad: there
are great interspaces, towards the hinder margin,
between the feathers. This, as I have shown above,
may aid automatically the maintenance of equili-
brium, but the gaps are wider and deeper than are
necessary for this purpose. In fact the Hoatzin's,
the Jay's and the Hoopoe's wings suggest the
work of a " 'prentice hand."

The narrow wing is certainly not primitive. I
have shown above (see Chap, n) that most of the
work is done by the front part of the wings, and this
becomes increasingly true when the wing moves
very much forward, cutting the air at a small angle
with the horizon. This is the way in which the long
wings of the best flyers cut the air. And it is in
these best of flyers (I am not speaking of the Soarers)

96 THE FLIGHT OF BIRDS

that we find the breadth of the wing much reduced ;
they taper towards the extremity where the rapidity
reaches its maximum and the forward movement is
greatest.

Styles of Flight.
There are two contrasted styles of flight that even
the casual observer easily distinguishes. There is
that of the small bird that lifts himself with two or
three rapid strokes, then takes a rest and glides
onwards, his wings as a rule not quite at their
full span — glides so far that though his pace is con-
siderable he loses not a little altitude. But a few
strong strokes soon make this good, and he enjoys
another slightly -downward glide. The Swift and
the Swallow are first-rate exponents of this style,
but it is common to most small birds. The Water-
Ouzel is a striking exception ; his wings move so
fast that you see only a blur, and he allows himself
no easy intervals of gliding. Probably this is
because, for his size and weight, his wings are
decidedly small, so that he has to ply them unceas-
ingly. When a larger bird flies in the dipping style
— a few strokes and a glide, a few strokes and another
glide — it is particularly striking. Everyone knows
the dipping flight of the Woodpecker. When he
glides, he flexes his wings more than most of the
birds that intersperse little glides amid their flight,
and so he dips more than others. Sometimes a bird
whose wings are large for his size and who gains
much altitude with a stroke or two, then half folds
his wings, has a very butterfly -like appearance. The
Wallcreeper, when he plays about a vertical face of
rock in the Alps, spreading and then half flexing his

VARIETIES OF WING AND OF FLIGHT 97

wings with their rich red coverts, looks like a butter-
fly, and a butterfly of marvellous beauty.

The reason that birds of larger build do not fly in
this style is, probably, as I have pointed out above,
that their rate of stroke being slower they lose
altitude while raising their wings, unless they have
considerable momentum. Their best policy, there-
fore, is to attain and maintain great pace, rather
than to gain altitude and then indulge in a slightly
downward glide. Moreover, if a big, bulky bird
were to set his body at a suitable incline for rising
almost vertically, as a Pigeon sometimes rises, his
wings, turning stiffly as they do at the shoulder-
joint, would not be able to beat in the right direction
and would drive him backward rather than lift him.
Some rather big birds do a good deal of gliding, but
it is a very different performance from that of the
small bird. They first gain altitude, not, however,
by only two or three strokes, as the small bird does,
but by a number. In fact they get up momentum
and then are lifted, as an aeroplane set at a slight
incline to the horizon is lifted when it is driven
rapidly forward. They will then glide onward with
wings outstretched to the full, so as to lose as little
altitude as possible. A wide spread of wing is the
thing needed, for, since it is the front part of the wing
on which the air mainly acts, the greater the front
presented the greater the supporting power. The
big bird's gliding is very unlike that of the small bird
that knows how easily by three or four strokes he
can recover elevation, and therefore quickens his
glide by the sacrifice of some of that which he
has already gained. Let us take as examples of

98 THE FLIGHT OF BIRDS

comparatively big birds the Grouse and the Partridge.
The great concavities beneath their wings, no doubt,
aid them much. But the intervals of gliding,
depending as they do on the preliminary attainment
of great momentum, necessarily come at rarer
intervals than they do in the case of the small bird.
I am not speaking here of the long glides achieved
without loss of altitude by some large, or fairly large,
birds when the wind has an upward trend. That is
a feat of the same nature as soaring, and I reserve
it for another chapter.

In another way the big and the small bird present
a striking contrast. The former is often the slave
of his own momentum, and only with difficulty and
effort deviates from his line of advance ; the Goose
and the Duck are good examples of this. The
Swallow, on the other hand, turns with the utmost
agility and ease. Some bigger birds have a wonder-
ful nimbleness, though, in this respect, I think, they
are no match for the Swallow. The Sparrow-Hawk,
as he pursues some coveted small bird, turns and
twists among the trees and bushes with great skill,
his long, broad tail helping him very much in steering.
Birds of prey must, of course, have great nimbleness,
great power of suddenly checking themselves or sud-
denly changing their course ; otherwise they would
be unsuccessful as hunters, and would dash themselves
to pieces when swooping upon some intended victim
that was flying not far above the ground or a tree.
The Lapwing turns to account his power of putting
on the brake and making sudden turns by dashing
at his enemy, alarming him, then suddenly pulling
up and retreating only to make another dash. The

VARIETIES OF WING AND OF FLIGHT 99

great breadth of his wings at their outer extremities
may, as I have said, account for his well-known
tactics and the skill with which he carries them out
(see PI. xiv).

The pace of the stroke varies very much in different
species, a subject on which I have already said
something.* A Stork goes along in very leisurely
style, taking no more strokes per minute than a
Heron, i.e. some 130, or even less. But one must
not imagine that little force is being used. The
Stork's wings are very long, and the upward bending
of the primary feathers shows the great rapidity
with which they are being driven through the air.
It is the birds with big, long wings that have the
slowest stroke. The great length of wing makes
each stroke very effective, and the slow beats that we
see so easily are not at all slow towards the wings'
farther end. Heavy birds that have short wings
have to take very rapid and very long strokes ; the
wing is lifted fairly high in horizontal flight, very
high when they are rising, and descends till the tip
has described a large segment of a circle. The Duck
is a familiar instance of this style of flight. His long,
strong strokes send him hurtling through the air
with ponderous momentum. The same style of
flight is carried to its extreme by the Guillemots,
Razorbills, Puffins, and other diving birds which use
their wings in swimming. It would be no good trying
to fly under water with big, long wings ; after the
completion of a stroke, to get the wing back into
position for the next would be a difficult operation.
These birds, then, have the largest wings with which

* See p. 50.

H 2

100 THE FLIGHT OF BIRDS

it is possible for them to swim, and the smallest with
which it is possible for them to fly. The wing-beats
during flight are marvellously rapid ; there is visible
a rapidly-moving body and a blur on either side
where the wings are working with terrific speed.
The Water- Ouzel searches for his food among the
weeds in the bed of fast-running streams, and he,
too, has short wings that he plies with very great
rapidity.

The rolling flight of Partridges and Grouse is very
striking. When you put them up, they never, till
they have got some distance off, keep on an even
keel. Probably this rolling flight makes it less easy
for big birds of prey to swoop down upon them with
true aim, and it may help them to glance at a
pursuing enemy. In fact, the Partridge and the
Grouse are still, in their habits, adapting themselves
to an environment of raptorial birds rather than to
an environment of sportsmen and gamekeepers.
The zigzagging flight of the Snipe — a very spirited
and characteristic performance — is a different thing ;
the line of flight is a zigzag, but at the same time
there is the rolling that we see in the Grouse's flight.
It is well calculated to confuse the aim of an assailant,
whether he be a bird of prey or a novice with a gun.

The way in which the long-necked and the long-
legged birds carry their necks and their legs is inter-
esting. The Duck and the Goose stretch their necks
forward to their full length. This may be because
the breastbone that carries the heavy flight-muscles
is very long, so that the head and neck are used to
balance the weight farther back. The Heron flies
habitually with his neck bent — a remarkable fact

VARIETIES OF WING AND OF .FLIGHT 101

if we bear in mind that he carries his long legs
stretched out behind. But his breastbone is short
and deep, so that the great weight of muscle lies
forward. Occasionally he may be seen to extend
his neck full length, apparently for balancing pur-
poses. The Stork is long-legged and the Flamingo
still more long-legged, and both of them carry their
necks and legs straight out. This is the normal
attitude also of birds of prey. The Stilt, too, flies
with his marvellous legs streaming out behind him ;
his style of flight is well shown in a drawing in Mr.
Abel Chapman's Wild Spain.

Flight in Flocks.

When birds fly in a flock, great or small, they often
adopt a particular formation, very commonly that
of a V, and this is sometimes spoken of as if it had
some special merit. But really the only important
thing for each bird is to keep clear of the wash of
his predecessor and the broken columns of air that
he leaves behind him. The force of the wash is the
measure of the vigour a bird puts into his wing-
strokes ; it will not do, therefore, to travel close in
the wake of another bird. The bicyclist, on the
other hand, breaks the resistance of the air, and the
man who rides close behind another has an advantage
since he finds the air more yielding. As the man in
front is not riding on the air there is no back-current
behind him (see p. 4).

The Whir of Wings.

Everyone must have noticed the different notes
given out by the wings of different kinds of birds as

102 THE FLIGHT OF BIRDS

they fly. Very striking is the contrast between the
shrill whistling of the wings of the slow-flapping
Swan and the whir or swishing sound that accom-
panies the flight of a flock of Starlings. It has been
thought that the particular note depended on the
pace at which the wing moved through the air, but
in reality it is due to the vibration of the feathers.

Wing music is often very beautiful ; it is a grand
thing to get near to a large flock of Golden Plover
and hear and see them go scudding by. There is
also pleasure in watching an Owl when he comes
near, and in realizing the complete silence of his
flight. At the edge of the outer web of his first
primary wing-feather, the hooklets of the barbules
are missing, and the barbules themselves are mere
vestiges. Consequently the edge of the feather has
the softness of down, and this, no doubt, has much
to do with the ghost-like silence of the Owl's flight.
The Nightjar also is a very silent flyer, though in
his wing -feathers the hooklets are nowhere missing.
Still his plumage is remarkable for its softness, and
this probably accounts for the absence of whir as
he plies his wings.

CHAPTER IX.
PACE AND LAST.

EXPERIMENTS AND OBSERVATIONS — WIND — VELOCITY OP
MIGRATORY FLIGHTS — ENDURANCE.

Many and various are the ways that have been
tried of measuring the velocity of the flight of birds,
and, unfortunately, the various ways lead us to
divergent conclusions. Some years ago some experi-
ments of indisputable accuracy were made in a
range constructed for experimental shooting. Two
" screens " formed of very fine threads were put up
at a distance of forty yards from one another. These
screens were connected with electrical apparatus,
by means of which the time occupied by the bird
in traversing the forty yards was registered. The
highest speed attained by any of the twelve Pigeons
experimented on was 33*8 miles per hour, the lowest
26*1. Similar experiments have been made in the
open. The velocity of four Pigeons was measured
on a calm day by persons stationed at a certain
distance from one another, who marked carefully
the moment at which the birds came opposite to
them and registered it with a stop-watch. The
fastest travelled at the rate of 27*9 miles per hour.
Pheasants were experimented on in the same way
in the range and in the open ; in the former case the
velocity was 33*8, in the latter 36*1 miles per hour.

104 THE FLIGHT OF BIRDS

These outdoor observations may be inaccurate, but
we can hardly cavil at the experiments in the shooting
range.* The verdict, is, indeed, a startling one. If
this is all they can do, birds have a reputation for
far greater pace than is warranted by the facts. They
are thought to be far swifter than the swiftest horse,
but Ladas's time over the Derby course, 1| miles,
gives him a velocity of 32f, while Spearmint in 1906
won the Derby in 2 minutes 38 '8 seconds ; i.e. he
maintained a speed of just over 34 miles per hour.

However, a thoroughly competent observer, Com-
mander H. Lynes, has made similar experiments
which give decidedly different results. In his
observations on the Migration of Birds in the Mediter-
ranean {British Birds, Vol. in), he writes : " The
only passage speeds I was able to deal with were
those of some of the species which arrived flying
low. The best observations were made on the
Quails by timing them from the moment they crossed
the fore-and-aft line of the ship to the moment that,
with a pair of glasses, they could be seen to fly into
a quail-net exactly 500 yards distant. The result
gave a speed of just 50 knots per hour. Corncrakes,
Water-Rails and Spotted Crakes arriving appeared
to be going just about the same speed, but proper
time-observations of them were never obtained."

Fifty knots is just over 57 miles, a very different
verdict from that pronounced on Pigeons by the
experiments in the shooting-gallery and the other
similar experiments in the open air. And yet the

* See Charles Lancaster's Illustrated Treatise on the Art of Shoot-
ing , p. 175, and Sir R. Payne-Gallwey's Letters to Young Shooters,
p. 152.

PACE AND LAST 105

Quail, when one sees it on migration, flying past a
steamer, does not seem to be one of the fastest flyers.
Unfortunately, Commander Lynes does not record
any observations made on the wind at the time the
Quails were flying the measured distance.

Mr. H. H. Clayton, when engaged in measuring the
heights and velocities of clouds by means of special
theodolites, took the opportunity of finding the pace
and the height at which passing flocks of birds
were flying. In one case — the birds were Ducks — he
found the speed to be 47 '8 miles per hour and the
altitude 958 feet above the ground. Some Geese
flew at 44*3 miles per hour at a height of 905 feet.*
I believe Mr. Clayton gives no record of the wind.

The records of the races of Homing Pigeons give
a very favourable verdict. But when the " times "
have been very good, there has been in almost every
case (perhaps in every case without exception) a tail-
wind to help them. I take some records from the
Homing Fancier's Annual for 1892. For 82 miles a
bird maintained a velocity of just over 71 miles per
hour. The weather was ' ' splendid, ' ' but the winner so
outstripped all the others that we cannot help being
slightly suspicious as to the correctness of the record.
In a longer race — 2 15 J miles, from the Scilly Isles
to its home in Wiltshire — a Pigeon kept up a speed
of 50 \ miles per hour. In a short race — only 80
miles ! — we have a velocity of 58| miles recorded.
In a race of moderate length — 170 miles — the winner
travelled at the rate of just over 54 miles. There
is another record of 57^ maintained for 104 miles.
In all these cases, with one exception, it is recorded
* Science, n.s., Vol. v, No. 105, p. 26.

106 THE FLIGHT OF BIRDS

that there was a tail-wind to help the birds. With
regard to the 80-mile race, the only meteorological
note is " weather hazy." In the Working Homer,
a high authority lays it down that a tail-wind is not
absolutely essential to good " times " ; what is all-
essential is ant icy clonic weather. A cyclone is
disastrous to them. In a light breeze during an
anticyclone it is recorded in this treatise that a
Homing Pigeon flew from Banff to North Hants at
the rate of 1,900 yards per minute, or 62 miles an
hour, an astonishing pace. The author tells of a
celebrated bird " Vonolel," which in two races
maintained a velocity of over a mile a minute.
Unfortunately he gives no record of the weather.

In France the experiment has been made of
employing Swallows in place of Homing Pigeons.
The idea is a very ancient one, for Pliny tells us that
a certain Roman knight, who wished to let his friends
atVolaterrae (in Tuscany) know who had won the
chariot races, used to take with him to Rome — a
distance of 130 miles — some Swallows, which he let
loose after dyeing them the colour of the winner.
Of the experiments in France I have not been able
to obtain any accounts at first hand. One flight is
reported to have been a very grand one, far sur-
passing anything credited to a Homing Pigeon. A
Swallow was taken from Roubaix to Paris, a distance
of 258 kilometres, or 160 English miles, and in 90
minutes from the time of its liberation at Paris it
was back again. It had kept up a pace of 106 miles
per hour ! * This may seem incredible, but the

* See an article quoted from the Globe in the Zoologist. for 1887,
p. 397. In the Homing News for Sept. 13, 1889, is an account
apparently of the same flight, the distance being given as 250
kilometres.

PACE AND LAST 107

figures may possibly be correct. It must be remem-
bered that the Swallow is better built for rapid flight
than the Pigeon. Of the velocity attained by the
Swift, that in his flight is very like the Swallow, but
certainly more than his match, many people have
arrived at an even higher estimate.

Wind.

But how are we to account for the Pigeon's com-
paratively very poor pace when tested in the shooting-
gallery ? I have shown that in most cases, when
they make records, Homing Pigeons are aided by a
tail-wind. Besides this it must be remembered that
at a considerable altitude the air is more rarefied,
and consequently offers less resistance. It is true
that it gives less support. But a bird of strong
flight, travelling fast, will get the support that he
requires, so that he will gain and not be hindered
by the rarefaction of the air. It hardly seems,
however, that this can be the sole explanation, since
Commander Lynes found that Quails maintained a,
velocity of 57 miles per hour when they had left the
higher air and, on nearing a resting-place on their
voyage, were flying low. Commander Lynes's
observations were made, I believe, on a number of
Quails under varying conditions, so that it seems
probable that we have here the bird's own pace, not
its pace plus that of the wind.

The difficult complication introduced by the wind
has been ably dealt with by Dr. Thienemann at his
post of observation not far from Rossi tten, the well-
known ornithological observatory on the Baltic.
He has set up two lines of stakes half a kilometre

108 THE FLIGHT OF BIRDS

apart ; they run at right angles to the usual stream
of autumn migration, which flows from NN.E. to
SS.W. He takes up his position at a point in one
of these lines ; at the corresponding point in the
other line his assistant is stationed. With a stop-
watch he marks the moment at which a bird passes
his post of observation : at the moment it passes the
other post, the assistant telephones the fact to Dr.
Thienemann, who records it on his stop-watch. The
direction of the wind is taken and the exact angle
it makes with the bird's line of flight ; its pace is
measured by means of an anemometer. With these
data, whatever the angle may be, it is not difficult
to calculate the distance travelled by the wind with
the bird or in the opposite direction, while he is
covering the measured half kilometre. After finding
the rate at which the bird travels per second, per
minute, per hour, Dr. Thienemann subtracts or adds
the distance travelled by the wind, with or against,
during the same time. Thus the disturbing wind-
factor is eliminated and he arrives at the bird's own
velocity. All this is wonderfully ingenious and
wonderfully thorough ; the results have great value
because they are undoubtedly dependable. We can
only regret that the species of birds observed do not
include any of those observed by Commander Lynes.
Moreover it seems probable that they were not flying
at full speed. Migration, as a rule, goes on at a great
height ; these birds had descended to a low level,
and with land beneath them were flying probably
in leisurely style. The much greater velocity of
Commander Lynes's Quails is accounted for, I believe,
if we bear two facts in mind ; they are, presumably,

PACE AND LAST 109

birds of more rapid flight, and besides this, though
they had descended from their lofty heights, they
were, unlike the birds at Rossitten, still straining
towards the land. But in any case Dr. Thienemann's
observations, in point both of method and results,
are interesting and valuable. The pace of no less
than twenty Grey Crows flying at different times
under varying circumstances was measured, and the
average velocity, after allowance had been made for
the wind, was 50*04 kilometres, or 3T5 miles per
hour. Two Jackdaws had an average of 39*6 miles.
A Starling flew at the rate of 46' 5. Six Finches
averaged 33*0, two Crossbills 37' 5. This is about
what we should have expected. But these definite
observations are much better than the shrewdest
of guesses.

Velocity of Migratory Flights.

There is good reason to believe that birds while
migrating attain far greater velocity than they do
in their ordinary flights. Let us take an example
for which there seems to be strong evidence, though
it is almost too marvellous to be true. The American
Golden Plovers breed in Arctic regions, from Alaska
to Greenland, above the limits of forest growth, and
when autumn comes they pass over Nova Scotia,
strike boldly out to sea, and, generally leaving the
Bermudas well to the west, sail on over the ocean till
they reach the West Indies. It is difficult to believe
that these are merely stray birds that have been
blown out of their course and are sailing on to death.
One witness after another declares that he has seen
flocks of them flying southward several hundred

110 THE FLIGHT OF BIRDS

miles to the east of the Bermudas, on which islands
they alight only if the weather is unfavourable.
Flying south from the Bermudas or somewhere east
of them, they must cover some 1,700 miles before
they land on one of the West India islands. Either
then they fly at an almost incredible pace or they
remain upon the wing an almost incredible time. If
this wonderful flight is really achieved by the
American Golden Plover, it is certainly the most
wonderful athletic feat with which birds can be
credited. But there are other flights which might
well strain our power of belief, if the evidence were
not so strong. There are migrant birds which pass
the summer (the Antipodean summer) in New
Zealand, and among them are some land birds,
notably two species of Cuckoo. " The long -tailed
Cuckoo " (Eudynamis taitensis) — I quote from Mr.
W. L. Buller's Manual of the Birds of New Zealand* —
" which is a native of the warm islands of the South
Pacific, visits our country in the summer and breeds
with us. It begins to arrive about the second week
in October, but it is not numerous till the following
month, when pairing commences." To get to New
Zealand from New Caledonia it must pass a very
wide stretch of sea, and, if it can find no small island
to use as a resting-place, it must cover 1,000 miles
in one flight. Since it almost certainly comes, not
from Australia, but from the islands to the north-
west and north of its summer home, there is little
Norfolk Island or the Kermadec Islands that might
be used as convenient halting -places. There is

* Buller's Manual of New Zealand Birds, p. 7. See Captain
Hutton's Animals of New Zealand.

PACE AND LAST 111

another Cuckoo (Chrysococcyx lucidus) called the
Shining Cuckoo, which also probably follows the
same route. Both of them pass over a very wide
but shallow stretch of sea, from New Caledonia to
New Zealand, a region that in the early Tertiary
period was dry land. These birds, ultra-conservative
like nearly all feathered creatures, are keeping up the
practice of a great migratory flight, which, when
first attempted, was an overland journey, but now
passes over hundreds of miles of open sea. Captain
Hutton and others who have studied New Zealand
migrants are, apparently, of opinion that Norfolk
Island and the Kermadecs are not used as pieds a
terre, at any rate by all the birds that flock southward.
And so they make the journey in one long flight.
That the land birds ever rest on the sea is extremely
unlikely, and it is, of course, impossible that they
should ever get food from it. 3 here is another
bird that certainly deserves mention here — the
Eastern Godwit (Limosa baueri). It has been
found nesting in Alaska and in Northern Siberia
(74°-75° N. lat.), and in August begins, in vast
numbers, to move southward, passing along the coast
of Formosa. It has been observed in Norfolk Island
— a very interesting fact. But do all the Godwits
reach New Zealand by this route ? Even if they do,
we must not assume that they always take a rest on
Norfolk Island. It may be that they only pause
there if the weather is unfavourable. It is highly
probable that some migrant birds cross from Australia
to New Zealand. The Australian Swallow occa-
sionally makes its appearance there, and even if it
pauses to rest on one of the Lord Howe Islands it

112 THE FLIGHT OF BIRDS

has still a stretch of more than 800 miles of sea to
fly over. Thus the New Zealand evidence, though
it is not yet as complete as we could wish, justifies
us in attributing to birds quite extraordinary powers
of endurance.

I have already pointed out that the rarefaction
of the air at high altitudes makes it less resistant
and less buoyant, an advantage without any draw-
back. The migrant bird flying, say, some three,
four or five thousand feet above the sea level, will
not suffer from the attenuation of the atmosphere,
for, since he travels with very great velocity, it will
give him all the support he wants. But there is
another fact that must be borne in mind which may
make the achievements of migrant birds more
credible.

Homing Pigeons, as we have seen, make the best
times as a rule when they have a tail-wind to help
them. Migrant birds, on the other hand, very often
have, it is said, the wind on the shoulder or blowing
almost straight in their faces when they make their
flights. But we must not without evidence jump to
the conclusion that the wind at the altitude at which
the birds are flying is blowing in the same direction
as it does at our level. We have only to watch the
clouds to discover that often at no very great height
there is an upper current that is not following the
same course as the wind below. Fifty years ago
Mr. Glaisher made some investigations, by means of
balloons, that threw much light on the subject. He
found that, though the direction of the wind close
to the earth was sometimes that of the whole mass
of air up to 20,000 feet, yet at other times the direc-

PACE AND LAST 113

tion changed no more than 500 feet up. Sometimes
directly opposite currents were met with at different
heights in the same ascent, and three or four streams
of air were encountered moving in different direc-
tions.* But we have other more recent observations
at our disposal. During the last few years the upper
atmosphere has been investigated with a thorough-
ness never before attempted. Unfortunately most
of the papers written on the subject deal more with
the question of temperature than with wind direction.
Often, too, they are concerned with altitudes to
which no bird could possibly attain. But when we
have made these deductions there still remains in
the papers published by the Meteorological Society
much that is of the greatest interest to the orni-
thologist. I single out the records of a few striking
observations.

On June 23rd, 1909, kite ascents at Glossop
brought out the fact that while the surface-wind
was south, there was at an altitude of 2,460 feet a
south-by -west current, f A great deal is to be learnt
from Mr. Cave's Pilot Balloon Observations in Bar-
bados, Dec. 6-11, 1909. { Mr. Cave found that on
Dec. 8th, at an elevation of some 4,400 feet, the air-
current formed an angle of 60° with the wind below.
On Dec. 10th, at 6.10 a.m., at a height of 5,500 feet,
there was a change in the direction of the wind
amounting to 50°. When, therefore, the migrant
bird may appear to have the wind on his shoulder

* See Encyclopedia Britannica, Vol. i, p. 267 (" Aeronautics ").

f " Registering Balloon Ascents at Gloucester," June 23, 1909,
by W. Marriott (Meteorological Society's Papers).

% Published by the Meteorological Society.

114 THE FLIGHT OF BIRDS

or to be facing a head-wind, he may in reality have
the velocity of a tail-wind added to his own. We
know as a fact that migrant birds do fly at a very
great altitude. In 1880 an American ornithologist,
Mr. E. D. Scott, at Princetown, New Jersey, used
an astronomical telescope to watch birds passing
over the disc of the moon at night. To be in focus
a bird must be not less than a mile distant ; it was
assumed that he would not fly at a greater altitude
than 10,000 feet. Knowing the angle made by the
telescope with the horizon, Mr. Scott was able to
calculate the lower limit ; the birds were flying at a
height of not less than half a mile.*

Another observer, Mr. F. M. Chapman,f also in
New Jersey, made a similar use of an astronomical
telescope for ornithological purposes, and watched
262 birds crossing the face of the moon. Of these
233 were calculated to be flying at a height of not
less than 1,500 feet. It is remarkable that those
that were at a low level were flying upward, as if they
had not yet reached the stratum of air most favour-
able to flight. Some of the birds, as they passed
over the moon's face, were silhouetted so clearly that
Mr. Chapman felt confident that he succeeded in
identifying the species.

Mr. F. W. Carpenter, in 1905, J tried a much more
elaborate plan. Two telescopes, set at some distance
apart — different distances were tried varying from
ten to twenty-one feet — were directed upon the moon
during a night in May and again during an October

* Bulletin of Nuttall Ornithological Club, Vol. vi.
t The Auk, 1888, pp. 37-39.
% See The Auk, April, 1906.

PACE AND LAST 115

night. The eyepiece of each telescope was crossed
by hairs that divided the field into " octants," and
each observer had a chart of the moon divided into
corresponding octants. When a bird appeared in
the area covered by both telescopes, its course across
the face of the moon was immediately marked on the
charts by straight lines, and the hour noted. The
bird, in fact, had the honour of being treated as a
star ; the angle at the bird subtended by the line
between the two observers was calculated ; a
parallax was obtained, and very fairly accurate
calculations were made. In May no bird was
observed flying at more than 2,400 feet (less than
half a mile) above the ground ; the lowest was flying
at a height of 1,200 feet. In October the birds
ranged from 5,400 feet (over a mile) down to 1,400
feet. The calls of birds not far overhead were heard
frequently during the observations, and Mr. Car-
penter remarks that, probably, most flew lower than
those observed through the telescopes. I don't
understand this, since Mr. Carpenter says that
objects as near as 1,000 feet were distinctly visible.
If so, why did the low-flying birds not come into
view ?

It is quite possible, then, that migrant birds get
much assistance from the wind. Though they often
fly in such a direction that our low-level wind would
strike upon their shoulder or even blow straight in
their faces, yet a higher current may be shoving them
onward, for they fly at an altitude up to which it
frequently happens that the wind that sweeps over
the earth's surface does not extend. Is it not
possible that Homing Pigeons, though they fly lower

I 2

116 THE FLIGHT OF BIRDS

than most migrants, may sometimes get into the
higher current ? If occasionally they make very
good times, apparently against the wind, may not
the explanation be that they have got into a stratum
of air that was moving in another direction ? Mr.
Glaisher, as I have pointed out, found that the
direction of the wind sometimes changed at a height
of no more than 500 feet. A Homing Pigeon, when
he is starting, circles upward and takes his bearings,
looking out for landmarks in the direction of the cot
for which he is yearning, and it is possible that as
he rises he may sometimes find a favourable breeze
though the lower one is adverse. I suggest this,
since a high authority on Homing Pigeons maintains
that anticy clonic weather is the important thing,
not the direction of the wind. Looking through
the best records, however, I find that the low-level
wind has been such as to propel the birds and add
to their velocity. If they find themselves in a
uniform horizontal current travelling in the direction
in which they are travelling, there is no reason why
they should be conscious of the movement of the air,
unless, indeed, they mark the rapidity with which
they pass their landmarks on the earth's surface
and draw the inference that there is a tail-wind
adding its velocity to theirs ! They must, of course,
fly faster than the wind, or the air will give them no
support. Now, supposing their own efforts give them
a velocity of 35 miles an hour and the wind has a
velocity of 25, the two together make up the splendid
total of 60. At present I say nothing about the
question whether birds are able to fly with the wind
when it is blowing a gale. Somehow the belief has

PACE AND LAST 117

established itself that a bird would rather face a
gale than fly before it.

Endurance,

I have already to a great extent dealt with the
question of endurance. It cannot be separated from
the question of pace when one is discussing the great
southward flight of the American Golden Plover or
the voyage of the land birds to their winter home
in New Zealand, or even the flights of Homing
Pigeons. In 1892 the bird that won the great race
of the Manchester Flying Club from Nantes kept up
a speed of 35| miles per hour for a distance of 430
miles. He was flying for over 12 hours, during which
his flight muscles had no rest. I am assuming that he
did not stop to rest. If he did stop for any length
of time, his speed while he was on the wing must
have been much over 35 miles. The wind was
favourable, from the south-west, and the weather
was fine, but this does not make the achievement a
commonplace one. Another bird, when the Preston
and District Homing Society had their Nantes race,
flew home — a distance of 441 miles — at a rate of 36
miles per hour. A man considers 30 miles in a day
a long walk. What a horse without a rider could
do I don't know, but I feel sure it would be some-
thing far short of 400 miles.

In April, 1909, when I was on my way to Egypt,
some Herons which were making their spring -migra-
tion flight over the Mediterranean showed astonishing
endurance. When they first appeared — a flock of
rather over 20 — at 5.30 in the afternoon, they must
already have flown at least 300 miles from the coast

118 THE FLIGHT OF BIRDS

of Africa. We were at the time just south of the
Adriatic ; rain came on about ten in the morning,
and numbers of birds, finding themselves enveloped
in cloud and unable to see their way, descended to
the clear air near the surface of the sea and so became
visible. Throughout the day there were continually
fresh arrivals. Many accompanied the ship for
miles, flying round and round her, and in some
cases settling and resting ; otherwise they remained
continuously on the wing. When the passengers
had retired to their berths the Herons were still
describing circles round the ship. Next morning
they were gone, and the officers who had been on
duty on the bridge reported that ten of the number
had flown round and round till the sun rose and then
had gone off northward, having probably sighted
the west end of Crete. In all they must have been
at least 16 or 17 hours on the wing.

Migrant birds seem to have altogether exceptional
power of endurance. Mr. J. L. Bonhote has called
attention to the remarkable fact that when starting
for a long voyage they are exceedingly fat, whereas
they are thin when they reach their destination.*
They would seem to live on their own fat, as the
tadpole, when he is becoming a frog, lives by absorb-
ing the fat in his tail. This is a very interesting fact
which helps us to see how the thing is possible. But,
in spite of actual or possible discoveries, the lasting
power of migrant birds must always excite our
wonder. Supposing that one of them takes no more
than 130 strokes per minute — a very slow stroke —
then, if he is on the wing 12 hours, the flight muscles

* See Ornis, Feb. 1909.

PACE AND LAST 119

contract 93,600 times ! The red, stringy Depressor
muscle can claim more credit for these marvellous
flights than any of the others. The heart, the lungs,
the whole machine, must be very strong and in
perfect working order.

Sometimes birds make long flights, requiring great
endurance, in the course of their day-to-day life.
Of this I give an astonishing example, which is
vouched for by two highly competent observers :
" Another fact that well-nigh struck dumb the
authors was that Ducks shot at dawn at Daimiel are
found to be crop -full of rice. Now the nearest rice
grounds are at Valencia, distant 180 miles ; hence
these Ducks, not as a migratory effort, but merely
as incidental to each night's food supply, have sped
at least 360 miles between dusk and dawn "* — and,
we may add, are probably ready to do it again the
next day and the next. Such a trifle as 360 miles
seems to put no strain on the digestive apparatus
or any part of the organism. The heaviest meal is
dealt with easily and causes no torpidity.

Whatever help the wind may give, it would seem
to be a fair inference from the examples which I
have quoted that birds surpass other animals in
vitality. It is not for nothing that their normal
temperature ranges in the case of some species to
111° F., and even slightly over.

* Unexplored Spain, p. 187, by Abel Chapman and Walter Buck-

CHAPTER X.

WIND AND FLIGHT.

RISING — FLIGHT WITH THE WIND — UNDULATING FLIGHT WITH-
OUT MOVEMENT OF THE WINGS ADVANCE IN A DIRECT LINE

WITHOUT MOVEMENT OF WDSTG — ADVANCE SIDEWAYS IN A DERECT
LINE SOARING SOARING IN A HORIZONTAL WESD IMPOSSIBLE.

No incline, however slight, escapes the notice of
the bicyclist. He is as sensitive as a spirit-level.
In the same way there are many birds that detect
every up-current, even the most local, and make
use of it, and, no doubt, detect the down-currents
no less. When they have no serious business on
hand they will practise manoeuvres in the air, making
the wind, as far as possible, do the work that would
otherwise fall on their muscles. A wind with an
upward trend will often lift them as if they were
feathers and nothing else, provided they throw
themselves into the correct attitude and hold their
wings rigidly extended. Nothing but an upward-
blowing wind can do the whole work of lifting, but
a wind that is not of uniform velocity may be of
assistance. In order to profit by the inequality,
the bird must pass from a comparatively slow current
of air into a comparatively rapid one. There is
hardly a bird possessed of the power of flight that
has not the skill to turn to account the fact that the

WIND AND FLIGHT 121

wind near the earth's surface increases in velocity
with altitude.

Rising.
Almost everyone has noticed that birds always
face the wind when they rise. In the case of big,
heavy birds this is particularly striking, for they
will often fly some little distance in the wrong direc-
tion, in a direction in which they certainly do not
wish to go, in order to get the help of the wind in
rising. When they have gained some little altitude
they turn and make for their objective. When a
steamer disturbs a Gannet floating on the water, if
there happen to be a fresh breeze blowing from the
steamer towards him, he will in rising head towards
the imagined enemy, and not till he has at his
disposal a few feet of altitude will he turn and make
off rapidly with the wind behind him. Oyster-
catchers will do the same thing. Once, when walking
along the sands south of the Solway Firth, I saw
hundreds of them in front of me. There was a
strong breeze blowing from me to them. Hence it
was much easier for them to make a start if they
flew towards me till they attained some slight
elevation. They therefore flew a little way towards
the disturber of their peace, then turned and settled
some way off upon the sand. I put them up a good
many times, and each time they began by flying a
short distance towards me ; so important is it for
a bird to get the help of the wind in rising. I once
saw a Cormorant fly a quarter of a mile or so in the
wrong direction. He had been feeding with his
fellows, which had all, after the meal, retired to a
rock, where they were drying their wings in the usual

122 THE FLIGHT OF BIRDS

Cormorant style. There was a fresh breeze blowing
from the feeding-place to the rock. As a preliminary,
therefore, he flew some considerable distance in the
opposite direction, then turned and joined the others.

If the wind is to help the bird to rise he must have
inertia ; in other words, he must not drift with it like
a balloon, but offer resistance. When he faces the
wind and takes his initial jump, obviously he has
inertia, for he has just left the earth, which is not
moving with the wind. Facing the wind he gets it
to help him. Imagine what would happen if he
faced in the direction of the wind ; impinging on
his back it would drive him downwards to earth
again. But the assistance got from the wind does
not end with the first jump into the air. The wind
rapidly increases in velocity with altitude. During
a terrific blizzard I once saw some Gulls with effort
making headway against the blast ; they succeeded
only by flying so close to the ground that their wings
almost touched it. Even when there has been no
gale blowing, but only a fairly stiff breeze, I have
noticed that Gulls, heading against the wind, will
fly as low as possible. Friction reduces the wind's
velocity. Some years back some friends and I called
in the help of an anemometer in order to get more
definite evidence. On one occasion the anemometer
recorded a velocity of 770 feet per minute at a
height of 2 feet from the ground ; at 7 J feet the
velocity rose to 1,000 feet.

Here are facts of the utmost importance to bird
and to aviator. We may for the sake of clearness
divide the air into distinct successive streams, the
second more rapid than the lowest, and each, as we

WIND AND FLIGHT 123

ascend, more rapid than the one below it, the rate
of increase growing less as the earth is left farther
and farther below. As he mounts through each
successive stream of air the bird has always inertia ;
he is never, like a balloon, the toy of the breeze. And
this, not only because he keeps plying his wings, but
because he is always emerging from a slower stream
of air into a more rapid one. Consequently, quite
apart from his vigorous wing-strokes, he offers
resistance to the wind ; he has, in fact, the inertia
that is indispensable. The force of the horizontal
wind is broken up into two forces, one of which tends
to lift him. The Lark, that past-master in the art
of upward flight, always gets the wind to lighten
the work of his wings, even up to considerable alti-
tudes. Sometimes, when the wind fails, he will sweep
vigorously round in a wide circle and make the
velocity due to his own efforts to some extent take
the place of a wind. Big and small alike, all birds
are glad to have the help of the wind. The muscles
that lift their wings are by no means strong, but as
soon as they have got some way on the rush through
the air does the work of lifting. A big, heavy
Elevator muscle would, therefore, be a useless
encumbrance during horizontal flight ; it is best to
put all the strength into the Depressor. Not only
is the Elevator small, but, as I have shown, it is of
inferior quality ; it has not much last. All the more
reason, therefore, to use artifice in order to economize
effort in rising ; or, to put it more correctly perhaps,
birds have not developed high-class Elevator muscles
since their skill rendered them unnecessary. Big
birds require the help of the wind to lift them at the

124 THE FLIGHT OF BIRDS

start much more than small birds. I have twice
seen Snipe, if not, strictly speaking, rise, yet begin
horizontal flight with the wind behind them. This
shows what a good start a bird that has etrong and
fairly long legs gets from his first jump forward and
upward from the ground. Things are very different
with a Swift or a Puffin, owing to the shortness and
feebleness of their legs, or such a bird as the Condor,
that has no room when he is starting for the full
sweep of his great wings. It is bad starters like
these that are in difficulties when there is no wind to
help them.*

Flight with the Wind.

I have already pointed out that Homing Pigeons
make their best " times " when there is a fair breeze
blowing in the direction of their flight. And for
all birds that fly, not only for Pigeons, there is
every reason to believe that a tail-wind means rapid
travelling. But what if the wind be a gale ? Some-
how or other an idea has grown up that birds cannot
fly, or at least do not like flying, with a gale blowing
from behind them. It is curious how hard it is to
get actual positive evidence for or against. Can we
find competent observers who have seen birds racing
before a regular gale ? And who has seen anything
from which we may reasonably infer that a gale from
behind is an abomination to them ? Some people
are satisfied that they have settled this question by
remarking that " it would, of course, ruffle up their
feathers, and it is an undoubted fact that birds

* If there is anything of a wind, a bird, if flying with it, always
wheels round and faces it when he alights, and the prudent
aviator follows his example.

WIND AND FLIGHT 125

dislike this." By way of answer we may say that
an undoubted fact is no better than blank cartridge
if it does not apply to the particular case. It is true,
no doubt, that whereas a horse or a cow will always
turn tail to a strong breeze, a bird always stands
facing it. I have just seen some forty Starlings on
a dead poplar tree. All, to a bird, were facing the
breeze. Gulls or Oyster-catchers may often be seen
by the seashore all facing to the front like soldiers
when the word " Eyes front ! " is given. The
nictitating membrane protects their eyes from the
wind, and there is no tumbling of their neat plumage.
But would such a thing be likely to happen during
flight from whatever direction the wind was blowing ?
A bird must have some velocity of his own ; he
cannot be like a leaf carried by the wind ; he is too
heavy for such a method of travelling, and, were he
to try it, he would soon descend precipitately to
earth. But though he necessarily has velocity of
his own added to that of the gale, it might seem
possible that a terrific tail-wind might occasionally
cause difficulty. Though the bird's body outpaces
the gale, yet there may be moments when the gale
overtakes his wings. When the down-stroke is over,
the wings are moved backward as well as upward
preparatory to the next stroke, though in long-
distance horizontal flight the forward and backward
movement is not very great. The velocity of the
wind, too, varies from moment to moment, and
Professor Langley showed that the greater the
velocity the greater is the irregularity. Even a
steady wind proves to be gusty when properly tested.
If, then, the wings happened to be moving backward

126 THE FLIGHT OF BIRDS

just at the moment of a terrific gust, when the gale
was outdoing itself, they might possibly be unable
to move backward as they should. Certainly this is
imaginable, though I cannot help thinking that the
bird, though flustered for a moment, would cope with
the difficulty. It must be remembered that it is
only at slight altitudes above the earth's surface
that the wind is so capricious and irregular ; the
obstructions it meets with there make it a broken,
boisterous torrent. The migrant bird flying high
above us is in a calmer stream, however rapid its
onward sweep may be. What we want is positive
evidence. Being in difficulties some years ago about
this question, I was delighted when I came across a
paper read before a learned society in Germany on
this subject. The writer maintained that birds did
often fly with a gale blowing from behind them, but
that under these circumstances their flight is so rapid
that we do not see them ! And the learned society
printed his paper ! Commander Lynes has recorded
an interesting observation that might seem to throw
light upon the matter. A migrating Swallow was
flying in the wrong direction, northward, when he
should have been flying southward.* Apparently
a gale sprang up, a gale from behind, and the Swallow
in consequence, as it appeared, turned and was flying
against the wind, as if intending to return to the place
from which it had set out. It must not be forgotten,
however, that migration, as a rule, proceeds at a
great height, and that there the wind may not be
blowing from the same quarter as it is at our lower
level. At Alderney some years back I saw what I

* British Birds, Vol. in, p. 141.

WIND AND FLIGHT 127

attributed to unwillingness on the part of small
shorebirds, Ringed Plovers and Dunlins, to fly with
a gale behind them. It was a real gale. I could with
difficulty make any headway when I faced it and
tried to walk. The small birds kept flying from one
patch of sand to another (the patches were scattered
among the rocks). They never headed towards what
seemed to be their objective, if, in order to reach it,
they would have had to fly with the wind behind
them. They would first face the wind and gain some
little altitude, and then turning, so that they faced
at about a right angle to it, let themselves be swept
to the patch to which, apparently, they wished to
go. Of this I saw a good many instances. But we
cannot build much on such observations, and having
since that time had occasional chances of watching
similar manoeuvres in rather less violent, but still
very strong winds, I am unable to draw the conclu-
sion that I then did.

Birds are very fond of playing when upon the
wing, and a good stiff breeze or a gale gives them
fine opportunities. When nestlings they have very
little chance of playing ; they are too crowded in
the nest, and in the case of many species restlessness
would end in a fatal fall. But as soon as they have
the use of their wings, the representatives of many
species are never tired of aerial sport. No doubt
this in a sense is practice. The Swallow improves
in agility, and so is better able to catch gnats when
he wishes to catch them. For the time he is not
definitely on the hunt, but is simply enjoying the
evolutions that the wind makes possible. If any
one doubts this, let him watch Gulls circling by the

128 THE FLIGHT OF BIRDS

hour together above a cliff that gives the wind an
upward incline. There are no fish to be caught
there, nor anything material to be got by the per-
formance. They are describing spirals high in air,
and by their skill they get the wind to relieve them
of all hard work. No wonder they enjoy it. To
return to the Swallows, whose evolutions are
different. When there is a fairly strong breeze
blowing, the first thing is with its help to attain some
altitude ; they face the wind and it helps to lift
them. Then, taking advantage of their position,
they enjoy a variety of downward glides. Often they
will face the wind and glide downward sideways, their
course thus crossing the stream of the wind. If
the wind has an upward trend this may be done
without loss of elevation, as I hope to show later on.
They can, of course, glide downward in the teeth
of the wind. Sometimes they will glide sideways
in the direction of the wind. In this case there are
often two simultaneous movements. The bird glides
downward, head leading, across the stream of the
wind, but the gale sweeps him along sideways with
such velocity that the other movement is obscured.
I think this is the explanation of what I saw at
Alderney. The birds were practising a sporting
manoeuvre ; gaining a little elevation and then
letting themselves be carried sideways by the gale.

Some aerial evolutions that I have just been
watching — Swallows and one Swift were the per-
formers — included no straight a-head flight before
the wind : it was a strong wind, though not a gale.
But I do not infer a positive dislike of such a thing.
The manoeuvres consist of varieties of tobogganing,

WIND AND FLIGHT 129

which familiar sport includes for human beings one
thing that is not enjoyable, viz., the lugging of the
toboggan to the top of the slope again. This part
of the performance is laborious for the bird also,
unless he can get help from the wind. This help
he gets when he faces it. When he has risen high
enough, he can plane down in whatever direction
he may choose to go. If he chooses to go with the
wind, he generally chooses the sideways method
that I have described. If he were to go head leading
and if his line of descent made only a slight angle
with the horizontal, there would probably be a
ruffling of his plumage, the thing that he abhors. It
could not happen were he to take vigorous strokes,
though the backward movement of the wing for the
next down-stroke might imaginably, as I have shown,
be a difficulty. During the Swallow manoeuvres
some Starlings flew past with the wind directly
behind, at a considerable height, too, where the wind
must have been even stronger. Their pace was no
mean one, and they seemed to be suffering no kind
of inconvenience. If the wind had freshened to a
gale, would it have been no longer a help but a
hindrance ?

Since writing the above I have had a chance of
watching Rooks and Starlings flying in so strong a
wind that it might fairly be called a gale. The
Rooks made use of up-currents and rose to a great
height. Some of them flew before the wind, putting
in vigorous strokes occasionally in order to outpace
it. They seemed not to be in any way inconveni-
enced by it. Though Tennyson speaks of Rooks on
a wild, windy day being " blown about the skies,"

130 THE FLIGHT OF BIRDS

these Rooks never for more than an occasional half-
second lost command of their movements. The
Starlings were no less at their ease, so that, to my
thinking, this curious little problem of flight with a
gale blowing from behind is settled. Birds are
capable of such flight, and much enjoy it. But to
migrate in such wild weather would be a different
matter. It is no wonder that they do not choose
a dark, stormy night for a long oversea voyage.

Undulating Flight without Movement of
the Wings.

At one time I believed that this was possible when
the wind was blowing horizontally, but I now feel
sure that an upward trend is necessary. Take an
example that is often to be seen. The wind is
blowing at something like a right angle to the course
of a steamer, and the Gulls, which are following to
pick up any scraps that may be thrown into the sea,
soon recognise that there is a chance of saving their
muscles. There is an up-current of air on either
side of the ship, for the wind is, so to speak, bent
upward some little distance before it reaches the
obstruction. To start with, then, the Gull obtains
some slight elevation by means of a few strokes, then
he glides down a gentle incline in the direction in
which the ship is travelling. When close to the
water he turns and faces the wind, which, having
an upward trend, lifts him to the level from which
he has just descended. He then glides downward
again, and so the process goes merrily on.

Were the wind a horizontal one it would, no doubt,
help the Gull to rise, but it would not make all

WIND AND FLIGHT

131

exertion on the bird's part unnecessary, any more
than it does in the case of the Snipe that faces the
wind in rising.

There is no reason why a bird should not advance
in the same way in the teeth of a wind when the
conditions are favourable, and occasionally Gulls
may be seen employing this method behind a steamer
when the wind is directly or almost directly ahead.
As the vessel moves onward, there is a down-current
behind the stern, since the air rushes down to fill
the space just vacated by the advancing ship. The
down-rush of air strikes the water and, rebounding

A W

Fig. 25.

Flight at right angles to the wind, with motionless wings,
the direction of the wind.

W:

from it, forms an up-current a little way farther
back. Here, then, is an up-current extending only
over a very small area of water, but, since it moves
on with the steamer, it opens up great possibilities.
To start with, imagine the Gull flying a little in rear
of the up -current. He glides swiftly downward and
onward, and when near the water finds himself in
the convenient up -draught, which lifts him to his
former level, so that he is able again to glide down-
ward and make much headway. At the end of his

K 2

132 THE FLIGHT OF BIRDS

glide he generally hits off the up-current, but not in-
frequently he fails and has to put in a few strokes.
Small pieces of paper thrown over the stern show that
the up -current is not always at the same distance
behind the ship. Hence this method of advance,
though a very gay and lively one, has, whenever I
have observed it, been lacking in precision. How-
ever, when the conditions are not perfect, the Gull
may be trusted to make the best of a bad job. I
have just been watching some Gulls that were
following a small steamer. The wind was blowing
nearly at a right angle to the vessel's course. There
was an up-current available, but apparently an
unsatisfactory one, for there was never an advance,
except for a few moments, without very distinct
wing-beats. The Gulls faced the wind and, while
almost uninterruptedly beating with their wings,
advanced nearly sideways, right wing leading. Had
there been no up-current, such a style of flight would,
I believe, have been impossible. They must have
been inclining their bodies slightly downwards, from
the left side to the right, so as to induce movement
in the direction desired. I am quite aware that
birds, when they wish to advance at right angles
to an ordinary horizontal wind, make a half-turn
towards it, so as not to be swept out of their course.
But here was an instance of a much more complete
turn and an advance almost sideways.

Clever as the Gull is at such methods of advance,
the Shearwater, to my mind, is a yet more perfect
master of the art. He does not require any steamer
to help him. If only there are waves and a wind,
he has all the conditions that he wants. He keeps

WIND AND FLIGHT 133

gliding downward and onward till he is almost
touching the water, then suddenly he faces the wind,
showing his white under-surface, and is lifted a few
feet, then he glides downward and onward again,
again faces to right or left, and is again lifted
without any sign of effort on his part. It is
a weird performance, more impressive than the
Gull's.

There must be up-currents when required, other-
wise the Shearwater must be superior to physical
laws, and such a superiority we cannot concede.
The waves give to the wind the upward incline that
is wanted ; even a small obstruction will cause a
very considerable deflection (see p. 134). Advancing
at something like a right angle to the wind, he feels
an up-current as he is gliding downward and onward
and at once turns and faces it ; thus he gains altitude
and can begin another onward glide. And so he is
able in most methodical style to cover large tracts
of sea. Gannets may be seen employing the same
method in British seas, and, no doubt, Shearwaters
also, though I have seen them chiefly in the Mediter-
ranean, where they are common. As a rule I have
seen them advancing in this style at a considerable
angle to the wind, but they sometimes employ the
same method for an advance in the teeth of the
wind. There is no reason why the Shearwater and the
Gannet, having the waves and the resulting up-
currents to assist them, should not do what Gulls
do behind a steamer when there is a wind blowing
across the vessel's course — I am speaking of the
clever performance described above (see fig. 25,
p. 131). The Shearwater and the Gannet have at

134 THE FLIGHT OF BIRDS

their disposal up-currents at short intervals, and
these are just as serviceable to them as the travelling
up-current behind a steamer is to the Gull.

Before I go further it will be best to mention some
experiments I once made with a vane which, instead
of swinging round to show from what point of the
compass the wind was blowing, worked vertically
up and down and detected up-currents and down-
currents of air. For its large arm it had a thin piece
of deal, one foot long by six inches broad, and this
was exactly balanced by a lump of lead at the end
of the shorter arm. There happened to be at New
Romney, where I made these experiments, a number
of banks forming barriers of a very convenient
height. While standing on a bank only two feet
high, its tripod lifting it four feet above the bank,
the vane pointed decidedly upwards. Five yards to
leeward of a bank six feet high it indicated that the
wind blew downwards, making a large angle with the
horizon ; there was but rarely an upward gust. Ten
yards to leeward of the bank the direction was still
mainly downward, but with not unfrequent upward
movements. At twenty and at thirty yards' dis-
tance the wind came in wild gusts, as often upward
as downward. On the windward side of the bank
the results were no less remarkable. Twelve yards
to windward the vane was not quite steady, but on
the whole horizontal. At a distance of six yards
there were occasional upward swings ; at four yards'
distance there was a decided upward tendency, and
this though the bank itself presented only a very
gentle incline. These facts set one thinking. If a
bank only six feet high is capable of so much, what

WIND AND FLIGHT 135

splendid up-currents must mountain ridges put at
the service of a soaring Eagle !

Advance in a Direct Line Without Movement of Wing.
There is a feat perhaps more striking than any of
those already described, a feat which, nevertheless,
Gulls often achieve. A steamer is advancing against
a fairly strong wind which, if not absolutely a head-
wind, strikes the vessel at an acute angle. There
results a steady up -current over the stern of the
vessel, or slightly to one side or the other of the stern.
Poised on this up -current the Gulls hang in mid-air,
their wings held rigidly expanded. Only very slight
wing-movements, evidently for purposes of balance,
can be detected. Standing on the deck and watching
these Gulls one is irresistibly reminded of the poising
of the Kestrel high in air with wings held motionless,
when he finds a wind that is all that he could wish.
It is sometimes easy to forget that, unlike the
Kestrel, they do not remain in one spot, but that
all the while they are moving onward and, in fact,
keeping pace with the steamer. The Gulls, like the
Kestrel, are poising on an up-current of air, but
they give their bodies a rather different incline, with
the result that they keep travelling forward. The
diagram will explain this. The general incline of
their body and wing surfaces is slightly downwards.
Hence the upward-streaming wind not only main-
tains them in air or lifts them higher, but, acting at
a right angle, also drives them forward. Imagine
a bird with his body sloped much more steeply
downward. Obviously, the wind would then give
him a shove forward. What the Gull does is like,

136

THE FLIGHT OF BIRDS

but with a difference, to a simple downward glide in
still air. In the downward glide, the bird or aviator
has to obtain a certain downward velocity by the
help of gravity before the air will give him support.
In the method of advance I have been describing no
downward velocity is required, since the air has an
upward trend and resists the pull of gravity. Some-
times this method of making headway without any
motion of the wings may be seen in mountainous
countries, where it is sometimes practised by
feathered experts capable of nobler achievements

Fig. 26.
Illustrating advance, with wings held rigid, in the teeth of the
wind, the wind having an upward trend. B-D, the body of the
bird sloped slightly downward. The arrow represents the wind.
Its force acting along F-M is broken up into two forces
represented by S-M and R-M.

even than the Gull. In Algeria I once saw two
Eagles sail straight ahead against the wind for about
a mile and a half without moving their wings till they
reached a high mountain ridge, blowing over which
the wind had got an upward trend. Having done
their mile and a half, they came back with the wind,
beating with their wings, and then repeated their

PLATE XVI.

A and B : Gulls following steamer, with motionless wings. Photo-
graphed from the stern of the ship. In A they are almost overhead.
The fact that there are so many with their wings horizontal shows that
it is not a ca^e of ordinary flight. (See Chap. X.)

WIND AND FLIGHT 137

majestic performance, but this time at a higher
level. It is not often, I believe, that there is a
steady upward-trending wind extending over so
long a reach. In the Alps I recently saw an Eagle
perform a similar feat, but not on so grand a scale.

It is clearly possible for a bird to advance in this
way facing with the wind instead of against it. The
wind acts at right angles to the bird's expanse of
wing and body, and, except for friction, it will not
matter whether it blows from in front or behind.
If it has a sufficient upward incline, it will not get
between the feathers and ruffle them. But, as a
fact, one very seldom sees a bird advancing in this
way. Still I have occasionally met with examples,
though I doubt whether the wind comes quite
directly from behind ; the bird turns himself just a
little sideways. Circumstances are favourable when
the wind blowing at a very slight angle against a cliff
is deflected upward. The Gull then advances with
the wind, head leading, wings motionless. But as
I have said, there is, probably, always a very slight
sideways turn made, so that the body from back to
tail is not quite in line with the course of the wind.
Is this to avoid a disarrangement of feathers — that
bothersome question ? Whatever the explanation
of this very slight deflection, and though this method
of utilising an up-current is not so common as others,
yet it is important that it should obtain recognition.
It will help us to understand soaring when we come
to investigate that difficult subject.

The wonderful flight of the Albatross, his wings
with their spread of twelve feet or more held motion-
less, I cannot undertake to describe, as I have never

138 THE FLIGHT OF BIRDS

had the luck to see it. Sometimes he seems to hang
motionless over the stern of the steamer in the
style with which Gulls have made us familiar. As I
understand the story, the Albatross in Coleridge's
famous poem was poised on an up-current above
the ship's stern, presenting a big, steady target
impossible to miss, when the Ancient Mariner was
mean enough to shoot the unsuspecting bird. Some-
times, I am told, the Albatross sweeps majestically
downward to a point some way off from the ship,
his wings all the while outstretched. It seems that
he must be practising the same kind of manoeuvre
that Gannets or Shearwaters practise, in their com-
paratively humble way, when they advance without
a motion of their wings at right angles to the wind,
or, occasionally, with the wind or against it. Accord-
ing to accounts, the Albatross takes downward
sweeps on a gigantic scale. Those who describe it
say that his evolutions carry him far away from the
vessel. How, then, are we to provide him with an
up-current that will lift him without his having to
move his giant wings ? What but the waves can
deflect the wind for him, when he has planed down
to regions beyond the steamer's sphere of influence ?
The Albatross is as completely subject to physical
laws as any other bird, or as a man of twenty stone
weight, and we may depend upon it that, if he is to
rise in the air, it must either be by means of the
contraction of muscles and powerful wing-strokes
or else by the help of an ascending current of air.
Those whose good fortune it is to see this noble bird
at his play should watch him carefully and note all
the conditions, instead of merely gazing, as many

WIND AND FLIGHT 139

seem to do, in open-mouthed astonishment. There
is one question that, I believe, has been but little
investigated. In tropical seas there may be up-
currents rising from the heated surface, just as there
are from sun-scorched plains even in much higher
latitudes, up-currents sufficient to serve the turn
of the Albatross. But his evolutions are to be seen
in Antipodean regions where no such heating is
likely to take place. Myself I have little doubt that
the Albatross's art is only that of the Shearwater,
though, owing to the great artist's enormous spread
of wing, the effect produced is much grander. A
writer quoted in Flight (Feb. 3rd, 1912) describes
it thus : " The flight is generally near the water,
often close to it. You lose sight of the bird as he
disappears between the waves and catch him again
as he rises over the crest. ... He alters merely the
angle at which his wings are inclined." Why, it is
just in this style that a Shearwater sweeps down-
ward, and then, by the help of a wave and the
resultant up -current of air, regains all the altitude
he has lost !

Advance Sideways in a Direct Line.

To return to the Gull, a more commonplace, yet
intensely interesting subject. Often, when he wishes
to advance at right angles to the wind, he faces it
and travels with motionless wings sideways, or,
more correctly, almost sideways. As if thinking of
his objective, he inclines head and body very slightly
towards it. We are, of course, presupposing an
upward-trending wind ; the Gull is poised upon it.
If the left wing leads, then the wind must be blowing

140

THE FLIGHT OF BIRDS

not exactly straight in the bird's face, but very
slightly from the right. A very little shove is enough,
as the air offers no great resistance to so well-built
an aeroplane travelling at no more, at any rate, than
twenty miles an hour, and generally slower. A
man-made aeroplane is bound to move at a great
pace or the air will not support it. But in the case
we are considering it is the wind that must have the

7m) t

Fig. 27.

Gull gliding sideways, the left wing leading. The wind

(represented by the arrow to the left) is deflected upward

by the cliff.

pace. Poised upon the strong, upward stream, the
Gull goes gently on his way. Some years ago, when
I was at Port Erin, in the Isle of Man, it was inter-
esting to see the Gulls returning in the evening to
the little island called the Calf, at the southern

WIND AND FLIGHT 141

extremity of Man. The wind blowing from the west
struck against the cliffs and was deflected upward.
The Gulls, as they always do, saw their chance ;
here was a fine, effort-saving up-draught. Flying
to the base of the cliff, they were lifted to the top
and far above it. They would then turn and face
the wind, and, with the left wing leading, return to
their night quarters, their heads being inclined just
a little towards the south. Occasionally Gulls adopt
this method of travelling when the wind blows
almost at right angles to the course of a steamer.
They will hang over it and keep pace with it, their
wings pointing to bows and stern. The slight adjust-
ments that they have to make for balancing purposes
are unceasing, but they are easily distinguishable
from the strong wing-strokes of ordinary flight.

The two methods — advance in the teeth of the
wind and advance with one wing leading — pass into
one another. Obviously so, since, to get support
from an upward trending wind, the Gull either
faces it or much less often turns his tail towards
it. Hence the necessity of progression sideways
when he wishes to travel in a straight line at a
right angle to the wind. When he travels at a half
right angle to it, the line of advance will bisect the
angle between the axis of his body and his wing.
To put it less mathematically, he will be half-side
face towards his line of advance.

Soaring.

Perhaps a definition of soaring may be useful.
The word is used to describe the spiral ascent of a
bird in the air, effected without taking any strokes

142 THE FLIGHT OF BIRDS

with his wings. He gets the wind to lift him, and
as he rises he circles, or, more correctly, describes
a spiral or helix. It is a marvellous performance.
Had we not an unlimited capacity for getting used
to anything, we should be lost in wonder whenever
we see this splendid achievement.

Many of the larger birds are proficient at it —
Eagles, Vultures, Pelicans, Storks, Falcons, Kites,
Buzzards, Ravens, Gulls and others, all, even the
smallest of them, possessed of wings that have a
very considerable area, and are very different in
outline from the long, narrow wings of the Tern or
the Swift. The Gull's wing is less definitely a soaring
wing than the others mentioned ; adapted both for
soaring and long-distance flapping flight, it is a
compromise between the broad and the narrow.*

Evidently breadth, and not only length, is im-
portant in soaring, and the great primary feathers
spread out, leaving very noticeable gaps between
them. Probably this gives steadiness, preventing
a too sudden escape of air from under one wing. If
a bird is watched through a field-glass or telescope,
the upward bending of these great feathers by the
force of the wind is sometimes quite noticeable, the
first primary being often bent considerably more
than the others (see the Frontispiece).

It is a slow, sedate movement, this circling high in
air. Mr. S. E. Peal,f who used to gaze with wonder
at the circling of the Adjutants, a kind of Stork noted
for their soaring, over the plain of Upper Assam,
held, if I remember rightly, that the birds slept as

* See PI. xiv.

f See Nature, Nov. 4, 1880 ; Sept. 26, 1889 ; May 21, 1891.

WIND AND FLIGHT 143

they described their sedate spirals. A bird is cer-
tainly capable of a great deal during sleep. When
he sleeps standing on one leg, he is perpetually
making small adjustments in order to maintain his
balance. When a Duck sleeps floating on a pond
with one leg tucked up, he will keep the other
paddling, so that he may move in a circle and not be
driven by the wind into the bank, where a hungry
stoat may be waiting for him. But to sleep while
soaring is an altogether different matter. The
soaring bird has not only to make perpetual adjust-
ments, but also to feel the pulse of the wind, to be
alive to every gust and find out what adjustments
have to be made. But the fact that so good an
observer could hold this theory shows how sedate
the movement is. Though the pace may vary, there
is not a rapid sweep down a gentle incline in one-half
of a circle, then, in the other half, when the bird has
wheeled round, a slow advance with much gain
of altitude : nothing corresponding to the gallop of
the four-in-hand down the last fifty or hundred
yards of a hill, in order that the coach's momentum
may carry it some way up the hill that is immediately
in prospect. The circling is slow, sedate, and appar-
ently perfectly comfortable, and sometimes certainly
the bird keeps rising through a whole turn of the spiral.
He does not sweep downward in one part, then turn
and gain elevation. If all goes well, if the wind is all
that is required, there is no loss of altitude from
beginning to end of the turn ; there may be a gain
throughout.

The birds that soar are all of considerable size
Small birds, however expert in flying, are, apparently,

144 THE FLIGHT OF BIRDS

unable to get the wind to lift them. And yet, rela-
tively to their bulk and weight, their wings are very
decidedly larger in area than those of big birds. The
explanation is, I believe, that, though the wing of
the small bird is relatively the larger, yet, actually,
it presents too small an expanse for the purpose of
soaring. The wind, instead of giving the required
support, escapes at the edges. In ordinary flight,
when the beating wings move with enormous
rapidity, their small area does not tell against them
as it does when the bird merely remains passive and
waits for the wind to strike it. In fact the velocity
of the wing's movement during the down-stroke is
distinctly greater than the velocity of the wind that
supports the soaring bird. Even a Heron or a Crow
in leisurely flight takes not less than 120 strokes
per minute, the Pigeon, according to Professor Marey,
480, the Duck 540, and the Sparrow 780. This
means that the farther part of the wing moves with
astonishing rapidity. The big soaring bird, more-
over, has a large cup-like concavity near the base of
the wing, which must hold the wind and so give
much support. Even the Gannet's wings, narrow
and elegant as they are, have near the body a deep
hollow that serves to catch and utilise any up-current
that offers.

Soaring always goes on at a considerable altitude.
In mountainous countries there are frequent oppor-
tunities of seeing it, and with luck one may occa-
sionally get near to the scene of the performance.
In Spain I once climbed to the top of a high cliff on
which was a Vulture rookery, some fifteen nests of
the Griffon Vulture, and saw the great birds circling

WIND AND FLIGHT 145

round a little below me. In hot countries it takes
place over wide plains where there are no hills
near. I have never seen or heard of it under such
conditions in cold, northern regions. Nor have I seen
any attempt at it over the sea. Gulls are no mean
performers at soaring, and they may frequently be
seen circling in fine style over cliffs. The fact that
they do not soar out at sea, and that they frequently
do where there are cliffs of any height, suggests the
secret of the whole thing. The bird when soaring
is lifted and maintained by an upward-trending
wind. At sea, in our northern latitudes, there are no
up-currents, or none of strength sufficient to make
soaring possible. Sir Hiram Maxim is able to detect
up-currents and down-currents of air that leave
smooth or ruffle our comparatively cold northern
seas.* It would be folly to deny their existence
without very definite evidence. It is very difficult,
however, to believe that they are currents of much
strength. The water far away from the Tropics
does not get heated sufficiently to cause a rapid
upward movement of air. Were the up-currents
which Sir Hiram Maxim has detected of any force
and lifting power, would not the Gull and the Shear-
water, quick as they are to avail themselves of any
little up -draught due to steamer or to waves, hasten
to make use of them ? I am quite aware that I am
here guilty, technically, of a petitio principii. I
wish to show that soaring depends on up-currents
of air, and I use the fact that Gulls do not soar at sea
— at any rate not over our northern seas — as evidence
that there are no strong up-currents. The reader

* Artificial and Natural Flight, p. 16.

L

146 THE FLIGHT OF BIRDS

must take the argument for what it is worth. But
there is direct evidence. Aviators are now exploring
the air just as birds for ages past have done, and I
read in an article on military aviation* that remous
(up and down currents of air) " are seldom met with
when flying over a uniform surface such as the sea."
In hot countries the air is heated by contact with the
sun-baked soil. It ascends and when, at some
height, it gets chilled by contact with colder air, it
forms cumulus clouds. It is found that the rate at
which balloons ascend varies much when there are
clouds of this kind about, and that is good evidence
that the air in places is streaming upward. f At a
low level there are no distinct upward and downward
streams, but at some height above the plains such
streams begin to form and the great cumulus clouds
tell us in what parts the movement is upward. One
day in the Nile delta, a very hot day with a blazing
sun, I was watching the Kites soaring. They were
wheeling and wheeling round and not a motion of
wing was to be seen. Suddenly a cloud obscured
the sun, and very soon all the Kites began beating
with their wings, and descended to a lower level.
It may be maintained that they do not care to soar
unless it is bright and sunny, and that they gave it
up because, there being no longer any sunshine, they
had no pleasure in continuing. Still I have seen
Ravens soaring over hills in cloudy weather, and
one day in the Alps, when snow or sleet fell at
intervals and the wind was raw and nasty, I saw a

* By Captain Brooke-Popham, Army Review, Jan. 1912, p. 88.

^ See "Methods for Observing Pilot Balloons," by C. J. P.
Cave, Journal of the Royal Meteorological Society, Jan., 1910.

WIND AND FLIGHT 147

Kestrel hovering with motionless wings. The up-
currents over sun-heated plains seem to have force
up to an enormous height. In Egypt I once watched
through my telescope a flock of Storks, ascending
with wings held rigid till they looked mere specks.
I wondered how they steered clear of one another,
there were so many describing mazy, intersecting
circles. It was just noon, and the day was decidedly
hot.

Of course there may be among mountains upward
currents that are in origin similar to those found
over level plains. The extraordinary heat of the
summer of 1911 has been well calculated to produce
such currents in unusual places. I have a letter
from Mr. R. C. Gilson that gives most striking
evidence of this. " Lying on my back the other
day," he writes (the letter is dated Sept. 16th, 1911,
" on the summit of the Murren Schildhorn, a flat-
topped eminence (about 10,000 feet) with pretty
steep sides — at all events two opposite sides are
steep, the mountain is ridge-shaped — I saw a piece
of paper carried up by the wind, and having no
tendency to descend, but the reverse. I then noticed
another much higher up, then others, an apparently
indefinite series (the mountain is frequented by
untidy tourists), of which the farthest that I could
see were mere silvery specks in the sunshine. How
high they were it is impossible to say, but I guess
not far off 1,000 feet above the hill. The weather
was anticy clonic, almost windless. Presumably the
elevating forces were convection currents from the
sun- warmed mountain-side. Where I lay I could
hardly detect a breeze, but there was always a very

L2

148 THE FLIGHT OF BIRDS

slight one perceptible if one approached close to the
edge of the hill."

What is going on in the air high aloft when soaring
takes place over a dead level I imagine to be this.
There is a wind sweeping over the plain, and at the
outset it is horizontal. Coming into contact with
an up-current from the heated surface below, it is
deflected upward. The soaring bird, then, gets
support not only from the ascending column of air,
but from the wind to which the ascending column
gives an upward trend. Were the heated air
mounting from below the sole support, birds might
soar in an almost dead calm, and that is a thing
which all observers agree does not take place. They
first ascend to some height— two or three hundred
feet — by beating with their wings, and then the
performance begins.

The reason of the spiral movement is not, I believe,
far to seek. There are over the plain regions of
ascending and regions of descending air. It is
essential that the bird should not pass beyond the
boundaries of the upward stream that maintains
and lifts him. Had the wind over the whole extent
of the plain an upward incline, then the Kites and
the rest might soar, if the term will stand this strain-
ing of its use, in a straight line like the Eagles, whose
majestic advance, without deviation to right or left,
I have already described ; or, to take an example
more commonly seen, like the Gull that follows a
steamer, poised on an up -draught over the stern.
The up-and-down currents on which birds depend
for soaring are sometimes very formidable to an
aviator, who in a few seconds may pass through an

WIND AND FLIGHT 149

ascending column of air and suddenly find himself
in a descending one. Captain Brooke-Popham,
whom I have already quoted, says that it is probable
that these remous seldom exceed 100 feet in width.*
In order to understand soaring, which often takes
place at great heights where it is impossible, at any
rate for anyone who is not an aviator, to investigate
air currents, it is well to consider whether it is in
principle at all different from what we may see taking
place close at hand where investigation is easy.
Gulls, as we know, have no difficulty, when the wind
has an upward trend, in poising upon it and making
headway against it. We have also seen that, the
upward trend being there, they will occasionally
advance, wings motionless, with the wind behind
or almost behind them. When a soaring Eagle
wheels round and circles, he does high aloft these
two things that the Gull does near to the earth. He,
of course, does something more, for when he makes
a complete turn of the spiral he must, in the course
of it, have the wind blowing first on one side of him
and afterwards on the other. But the Gull too does
something that approaches to this, for when he
glides sideways he is not absolutely full face to the
wind, but makes a slight turn so that it strikes him
a little on one side. The soaring Eagle faces each
point of the compass in turn, for he has to circle
round in order not to pass beyond the limits of the
upward stream of air that supports him. Let us
picture him as he turns the spiral. He does not,
like the earth, keep his axis pointed the same way
from the beginning to the end of his orbit. As he

* Log. cit., p. 88.

160 THE FLIGHT OF BIRDS

revolves, he faces north, west, south, east in turn,
or vice versa ; and, as he turns, the inner wing will
always be rather lower than the outer one — this is
with all birds the commonest way of steering.
How he changes his balance is not quite clear. He
may, holding his wings rigidly in a straight line,
pull his body towards one wing or the other, and so
weight one side more heavily than the other. More
probably he bends sideways at the waist, as birds
certainly do for balancing and steering purposes
(see p. 60). Whatever the method, he has to execute
slow, swaying movements with the utmost skill.
Only when he travels straight for a bit will the two
wings be on the same level. There is no reason why
he should be in difficulties at all when the wind
strikes him on the side. It will support him which-
ever way his head points. A glance at the frontis-
piece will help to make matters clear. Though the
wings are held rigid, they are seldom horizontal for
long together. The spiral movement demands a
continual swaying of the body ; for the correct
incline, not only fore and aft, but also from left to
right, has to be maintained. The tail is frequently
at work, and this is probably accompanied by the
other movements I have just described. Otherwise
the body would not swing round promptly ; the bird
would not be able to describe his airy spirals. Fre-
quently the wings are held slanting throughout the
" circle," the inner wing pointing downwards to the
centre and seeming to act as a pivot on which the
bird revolves. But this is not always the case.
Sometimes a bird will put in a considerable bit of
straight-ahead travelling, gaining, maintaining or

WIND AND FLIGHT 151

losing altitude as he goes. Not ^infrequently birds
change from a left-handed to a right-handed spiral ;
Gulls are very fond of this ; they often describe
very small " circles," revolving round the down-
ward-pointing inner-wing, and it may be that the
change to the right-handed spiral saves them from
giddiness.

Mr. Peal was of opinion that the Adjutants, whose
soaring he studied so zealously, always made leeway
as they rose. But there seems to be no reason why
this should be so. It may have been that what
looked like a movement to leeward had for its object
the keeping within an upward slanting stream of air
on which they depended. There seems no reason
for an involuntary loss of leeway. Let us consider
a particular turn of the spiral from start to finish.
When the bird is facing the wind, he has only to
incline his body correctly and he will make headway.
When the wind strikes him from behind, he must
slope his body downward, otherwise there will be a
disarrangement of plumage, and there will result a
slight loss by leeway, though no loss of elevation
unless the bird so chooses. When he turns his side
to the wind he is free to set himself at any incline he
likes, and, according as he chooses, he will advance
or retreat. During far the greater part of the circle,
therefore, he is far from being the plaything of the
wind. As a matter of fact birds may be seen circling
over hill-tops without any loss by leeway, and I
believe I have seen them equally successful over
plains. There is this, too, to be remembered. When
a bird has gained all the altitude he wants, by gliding
slightly downward he can make headway in whatever

152 THE FLIGHT OF BIRDS

direction he wants, and, if there has been any loss by
leeway, make it good.

Soaring in a Horizontal Wind impossible.

Given two things, a strong upward stream of air
and a big bird possessed of great skill, soaring
becomes quite explicable. But there are still some
people, I believe, who hold that an up-current
is not an indispensable condition, but that a hori-
zontal wind is all that is needed if only the wind is
not uniform, so that somehow the bird may perpetu-
ally manage to be passing from a comparatively
slow current into a faster one. How such a process
can be continued for an indefinite time is more than
I can understand. Nevertheless some great mathe-
maticians, who do not, however, profess to have
actually watched birds soaring, maintain that it
is theoretically possible. Setting aside theoretic
possibilities for the moment, let us see what we can
learn by observation. Wind is least uniform close
to the earth, and there we find birds turning this
want of uniformity to account. They face the wind
as they rise and get help from it, owing to the fact
that they are perpetually passing from comparatively
slowly moving air into a more rapid current. But
they cannot get the wind to do the whole work of
lifting, whatever onward momentum they may have.
They ply their wings vigorously all the time.

Evidently, then, a wind with varying velocities is
not enough to account for soaring. Imagine, too,
what would happen as the bird circled round. In
each complete turn of the helix he must, for part of
the time, have his back turned to the wind, and the

WIND AND FLIGHT 153

wind impinging on it — I assume the bird's body
would be inclined slightly upward — would drive the
bird downward, reinforcing gravitation with fatal
effect. Again, it has been imagined that he is
somehow perpetually passing from a slowly moving
to a rapidly moving part of an eddy, a very precarious
business. Even if we grant that such a method of
soaring is theoretically possible, yet the stately,
sedate wheeling of an Eagle shows that he is depen-
dent on quite different conditions. He finds a strong
up -current at his service, and he is securely riding
upon it. Practical aviators, if they have not already
done so, will be interested to read Mr. Wilbur
Wright's observations on soaring birds. He has no
doubt as to the necessity of an up-current. He has
often watched Buzzards soaring ; he calculated the
upward incline of the wind where it was deflected
by a hill, the hill where he and his brother were
practising with their glider, and he considered the
question whether he might not himself with practice
learn to soar.*

And now wonderful news has come from America.
Mr. Orville Wright has ascended on his glider, lifted
by an up-current, and for sixty seconds has hung,
almost without a quiver, in the air at a height of
seventy feet over a hill- top, a truly marvellous
achievement, worthy of a Kestrel.

* See " Gliding Experiments of the Wright Brothers," in the
Aero Manual, 1910,

CHAPTER XI.
SOME ACCESSORIES.

DIGESTION — CIRCULATION, BREATHING, TEMPERATURE — REPAIR
OF THE MACHINE CALL-NOTES AND SONG.

No one who undertook to describe a steam-engine
would ever dream of omitting all mention of the
furnace. To do so would be to leave out, if not the
part of Hamlet, yet a most important part. I have,
therefore, determined to write a short chapter on
certain " accessories," which would, perhaps, be
more correctly called preliminaries. These pre-
liminaries are matters of physiology, such as feeding,
digestion, breathing, regulation of temperature.

Digestion.

A bird is a glorified reptile, and one great difference
between him and his cold-blooded ancestors and his
cold-blooded surviving relatives also is that he needs
far more fuel to keep the flame of his life burning
with its normal brightness than they require to
maintain their slowly smouldering fires. The bird
has a huge appetite, and, except when a demand
for a prolonged, uninterrupted effort is made, he
craves for ample meals at no long intervals. The
boa-constrictor will go a week, or, in captivity, three
weeks or more without eating, even during hot
weather. But such abstinence does not suit a bird.

SOME ACCESSORIES 155

He has a temperature considerably higher than that
of a human being ; in some species it ranges up to
111° F., or even just over that. In fact his vitality-
is very great, and all his physiological processes are
brisk and vigorous. It would not do, therefore, to
economise in fuel. Though his fore limbs have been
metamorphosed into wings and are incapable of
doing the work of hands, his beak at the end of his
long, supple neck (for it is of some length even in a
comparatively short-necked bird) is quite equal to
the task, and picks up big and small things easily
and with skill. Moreover, birds of prey use their
feet as hands, and most effectively too. As soon
as he has seized his food, the bird sends it post-haste
to his crop, or, if he is cropless, to his stomach, his
proventriculus, to be digested. He has nothing cor-
responding to the apparatus that makes breathing
easy for us while we chew our food. Nor has he
any teeth ; powerful teeth could only exist if his
jaws were strong and heavy, and it is important
that his head should be light, for a heavy head would
make fore-and aft balance difficult during flight.
Digestion is rapid. In the case of a seed-eater, after
the digestive juices have operated on the food in the
proventriculus, it passes to the gizzard, to be ground
up in that powerful mill. There seems to be no
period of torpor after a meal as there is with a reptile,
though the actual weight of what is swallowed may
make flight difficult.

Circulation, Breathing, Temperature.
The heart is very efficient. Like the mammalian
heart, it has four chambers, with an impassable wall

156 THE FLIGHT OF BIRDS

separating the two on the right from the two on the
left, and the arterial blood is kept quite apart from
the venous. The valve between the upper and lower
chambers on the right is a single flap of muscle, very
unlike the three flaps of membrane found in the
mammal's heart. But it works in the same way, and
is no less efficient. The blood swarms with red
corpuscles, not round and without nucleus like ours,
but oval and nucleated. Where they come from
is a question. The marrow in human bones is
believed to be a factory of red corpuscles, and so we
cannot help wondering what substitute for this those
birds may have whose chief bones are hollow, with
only the very thinnest lining of marrow We know
that they have somewhere as good a factory as any
mammal has ; the richness in corpuscles banishes
all doubt on this point. It may be that the spleen
is very active. It is known that in mammals during
the embryo stages and in after-life, in emergencies,
the spleen gives birth to many red corpuscles.
Whatever their origin may be, there they are in their
thousands, putting vitality and energy into the bird.
The breathing apparatus is as wonderful as any
part of this living flying machine. The lungs are
very small and may be seen, brilliantly scarlet,
neatly packed on either side of the backbone. Their
efficiency is not to be measured by their very diminu-
tive size. They have large extensions called air-sacs,
into which the air rushes, passing through the lungs,
when inspiration takes place. Some of the air
breathed in finds its way at once into the minute
air-passages that ramify about the lungs and does the
work of oxidising the blood. The rest rushes straight

SOME ACCESSORIES 157

through large channels to the air-sacs, and, when
expiration takes place, passes again, still almost
fresh, to the lungs, and some of it, getting from the
main passage to the little ramifications, continues the
process of oxidation. Thus, if the bird takes twenty
breaths in a minute, the lungs are supplied forty times
with fresh air. If he has hollow bones — and in some
birds almost all are hollow — pouches of pulmonary
membrane extend into the cavities, and thus the
bones are filled with air from the lungs (see p. 77).

When a bird is standing his breastbone moves with
each breath ; in a captive bird this is easy to see.
In the case of Pelicans I have found the rate per
minute to vary from 5-11, from very slow to rather
slow, while in the most rapid breather, a Canary,
it was not far short of 100. Between these
extremes came a Blackbird with 39, a Bulbul with
48, an Ouzel with, at one time, 34, at another 50.
Evidently the big bird when at rest is a slow breather ;
a Griffon Vulture took only nine respirations. The
small birds are the more rapid, and as a rule they have
a higher temperature. The Great Tit and the Swift
are at one extreme with 111 2° F., and the Ostrich
at the other with 99'2°. In between come the Duck
with 109- 1° and the Heron with 1058°. The slow
breathing of the big birds during rest is remarkable,
but we cannot doubt that it becomes rapid during
flight, whether the bird be big or small. Unfortu-
nately it is very difficult to make observations, but
of the method of breathing we may get some idea
by watching a bird lying on his breast When he
adopts this attitude it is easy to see that his back
rises with each respiration, no movement being

158 THE FLIGHT OF BIRDS

possible for his breast since he is resting his weight
upon it. There is every reason to believe that he
breathes in the same way during flight, his backbone
rising and falling while his breastbone remains
steady. It is difficult for the breastbone to move
freely, since the pressure inwards of the wings tends
to hold it fixed. Were it to move easily it would
form a very unsteady pivot for the wings. The
lowering of the wing helps to lift the back, for as it
descends it hauls upon a muscle which passes from
the upper armbone to the backbone, and sometimes
even to the pelvis. Attempting to get direct evi-
dence of this method of breathing, I suspended a
freshly-killed pigeon by its wings and inflated its
air-sacs by means of a blowing-tube inserted in the
windpipe. The backbone, a little anterior to the
thigh-joint, moved rather more than half an inch, the
movement of the breastbone being almost too slight
to measure. Of course, the conditions that obtain
during flight were not reproduced ; there was no
pressure inwards. But the only result of such
pressure would be to render the breastbone and the
bones united with it still less ready to move.

The spacious air-sacs are useful not only for
breathing. The bird regulates his temperature, but
not by the machinery that is most effective in human
beings and most mammals, for, like his reptilian
ancestors, he does not perspire at any part of his
surface. During hard exercise he prevents a rise to
fever heat by giving off aqueous vapour from his
lungs, and besides this the great amount of air that
he breathes out when respiration is rapid has pre-
sumably a temperature not much below that of the

SOME ACCESSORIES 159

body. In other words, he is parting rapidly with
caloric, and the result is that his temperature is
regulated as effectively as any mammal's, though
the system is in the main unlike that which operates
in ourselves. All mammals, of course, throw off
aqueous vapour from the lungs, but in most of them
perspiration plays a very large part. Before passing
on to other matters we must note that the air in
pneumatic bones can be of little or no service in
breathing, since it cannot be expelled at will.

Repair of the Machine.
Very often unfair comparisons are drawn between
nature's machines and those which men manufacture.
It is forgotten that the former have not only to do
their special work, but also to keep themselves in
repair ; besides which they must reproduce them-
selves, i.e. they must be practically immortal. What
a contrast to this is presented by an aeroplane which,
without the constant attendance of skilled artificers,
is speedily reduced to helplessness ! Of the bird's
stoking I have already spoken. The repairing is
very wonderfully effected (see pp. 88, 89) without
the machine having to go into hospital. The great
wing-feathers on which flight depends and those in
the tail also are moulted in pairs, so that, though
not at his best while the process is going on, the bird
is at no time incapable of flight. A few birds, as I
have pointed out above, are exceptions, but under
their special circumstances Sightlessness , though,
no doubt, an inconvenience, does not as a rule bring
disaster. But for all warm-blooded beasts there is,
of course, a time of helplessness when they sleep and

160 THE FLIGHT OF BIRDS

recuperate after the labours of the day. For most
birds, however, the danger is reduced to a minimum.
While they sleep perched on a branch the weight of
their bodies keeps their legs bent at the ankle, and
when the ankle is bent the toes grip automatically.
Were it not for the automatic grip of the toes, the
bird would fall and be at the mercy of any prowling
enemy.

Call-notes and Song.

When the bird became able to move with speed and
make long journeys, some new power was required
to bring the sexes together and to prevent a pair
that had mated from becoming separated. Hence
it is that birds are loud-voiced, though reptiles are
nearly all of them dumb. To a mere call-note, a
very humble origin, it is probable that we can trace
the beautiful songs of the Nightingale and the
Blackcap. For many of the migrant birds in par-
ticular a loud, easily recognisable call-note was an
urgent need. After a flight of some thousands of
miles cock and hen must somehow find each other.
The Nightingale, when he has passed the Mediter-
ranean and reached his northern home, trumpets
forth to all the world the fact that he has arrived.
His future mate hears the call, and together they set
about the all-important business of nesting. And
thus we see that all a bird's activities must be viewed
in connection with the fact that he has wings and
is capable of long flights.

The End.

INDEX.

Adjutant Bird, 68, 142

Aero Manual, 18

Aeroplanes, 10, 12, 16, 18, 27,

36, 54, 61
Air, Rarefied, 112

Resistance of, 2-5, 6-22, 41

Albatross, 137-139
Alighting, 64-66 (PI. x), 124
Alix, M., 82
Angle of Incline, see Friction,

Gliding, Stability, Tacking
Archaeopteryx, 95
Area and Cubic Content, 19, 76
Aston, Mr. W. G., 18
Avanzini, Law of, 26
Aviators, see Aeroplanes

Backbone, 60, 62

Beetham, Mr. Bentley, 61, 65

Big Birds, 45, 55-58, 97, 98, 143,

144
Blood Corpuscles, 156
Bones, see Clavicle, etc.

Pneumatic, 76-78

Bonhote, Mr. J. L., 89, 118
Breastbone, 67-69
Breathing, 157
Brooke-Popham, Captain, 54,

146, 149
Buller,Mr. W. L., 110
Butterfly, 19
Buzzard, 153

Camber, see Curve
Carpals, 80

Carpenter, Mr. F. W., 114
Catapult (used to illustrate Glid-
ing), 26
Cave, Mr. C. J., 146
Chapman, Mr. Abel, 101, 119

Mr. F. M., 114

Circulation of Blood, 155

Clavicle, 69, 70

Clayton, Mr. H., 105

Cody, Mr., 61

Condor, 52

Coracoid, 69

Cormorant, 66, 121

Cubic Content and Area, 19, 76

Cuckoos in New Zealand, 110

Curlew {see PI. xiii), 32, 93

Curve of Wings, 15-18

Excessive, 25, 26

Darwin, 52
de Lucy, M., 19
Digestion, 154

Duck, 31, 32, 53, 62, 68, 98, 119,
143

Eagle, 57, 77, 136
Endurance, 117-119
Equilibrium {see Stability), 23-32

Falcon, 68

Feathers {see Pis. xi and xii),
46, 84-86

Moulting of, 88, 89

Rotation of, 82

Structure of, 86, 88

Feet, Use of, in Flight, 31, 32

Structure of, 90

Webbed, 60

Flamingo, 20, 31

Flight, Direct Advance without
Wing-strokes, 135 (PI. xvi),

in Flocks, 101

Sideways, without Wing-
strokes, 139

Styles of, 31, 96-102

Undulating without Wing-
strokes, 130

Velocity of, 103-117

with the Wind, 124-130

162

INDEX— continued.

Flight (Aero Weekly), 14, 16, 25
Fowl, Jungle, 73
Friction, 14

Gannet (see PL xiv), 65, 76, 85,

94, 121, 133
Gilson, Mr. R. C, 147
Glaisher, Mr., 112
Gliders [see Aeroplanes), 25, 26,

29, 153
Gliding, Birds, 1-22, 96, 97
Gnat, 19

Godwit, Eastern, 111
Goose, 88

Gravity, Centre of, 23, 24
Grouse, 73, 98, 100
Guillemot, 68
Gulls, 47, 60 (PI. ix.), 77

Play of, 128

Soaring of, 145

Progression of, without

Wing-strokes, 130-132, 139-

141 (PI. xvi)

Hawks, 33, 74, 98
Heart, 156
Helmholtz, 56
Heron, 31, 32, 50, 117
Hoatzin, 68
Hoopoe, 92
Hornbill, 78
Humerus, 71
Hutton, Captain, 110

Jay, 91, 93 (PI. xv)

Kestrel, 4, 41, 146
Kite-flying, 9
Kites Soaring, 146

Lancaster, C, 104
Langley, Prof., 12, 13, 125
Lapwing (PI. xiv), 32, 64, 66, 98
Legal and Reichel, 24, 32
Legs, Use of, in Balancing, 31,
32, 60

Structure of, 90

Lift and Drift, 1-15
Ligament, Elastic, 82 (PI. xi)
Lilienthal, 16

Lizard, 81

Lynes, Commander, 104, 126

Marey, Prof., 4, 5, 19, 24, 40. 43,

50, 144
Marriott, Mr. W., 113
Martin, House-, 35
Maxim, Sir H., 12, 145
Membranes, Wing, 85
Metacarpals, 80
Migration, 109-117, 160
Moor-hen, 75
Moulting, 88, 89, 159
Muscle, Quality of, 46, 55, 72-75,

81
Muscles 45, 55, 70-72
Weight of, 23

Newton, Sir I., 3, 12
Nuthatch, 65

Ouzel, Water-, 100
Owl, 102

Pace, 103-117
Parallelogram of Forces, 7
Payne-Gallwey, Sir R., 104
Peal, Mr. S. E., 142, 151
Pettigrew, Prof., 119
Pheasant, 53, 92 (PI. xiii.)
Pigeon (see Pis. ii, iii, iv, v, vi,
viii, ix, x), 19, 32, 46, 64,
97 103

Homing, 105, 106, 117

Pilcher, Mr., 29

Plover, American Golden, 109

Pressure, Centre of, 34

Pterodactyle, 49

Puffin, 48

Pygostyle, 33

Racehorse, The, 75

Radius, 80, 82

Rails, 89

Raven, 146

Reed-Warbler, Clamorous, 92

(PI. xv)
Robertson, Mr. J. B., 75
Rook, 129

I N D EX — continued.

163

Scapula, 69

Shearwater, 133, 134, 139
Snipe, 130
Soaring, 141-153

among Mountains, 144

always over Land, 145

over Plains, 146

impossible for small Birds,

143
impossible in horizontal

wind, 152
spiral Movement in, 148,

151
Sparrow-Hawk {see PL xiii),

32
Starling, Prof., 73

93 (PL xv)

Stability, 23-37

Pendulum, 25

Starting, 48-58 (PL viii), 121-

124
Steering, 59-63 (PL ix)
Sternum, 67-69
Stork, 19, 20, 101
Surface, Area of supporting, 18-

22
Swallow, 19, 20, 98, 106, 128

Australian, 111

Swift, 35, 48

Tacking, Boat, 8, 26 ')

Tail, use of, in Balancing, 31-34

Soaring, 150

Steering, 60

Temperature, Regulation of,

159
Tern, 64, 94
Thienemann, Dr., 107

Ulna, 80

Vane, Experiment with, 134
Velocity, see Flight.
Vertebrae, 76

Voluntary Adjustments, 29-37
Vulture, 144

Weight and Supporting Surface,

19
Wind and Flight, 107-109, 120-

153
Wind, at High Altitudes, 112-

117

Horizontal, 130, 152

not Uniform in Velocity,

122-124, 125
having an upward Trend,

130-153
Wing {see Area, Surface, Sta-
bility), Curve of, 15-18, 25,

26

Elasticity of, 27

as Lever, 38-40

Rotation of, 51-54

Shape of {see Pis. xiii-

xv), 91-96

Spreading of, 81-86

Weight of, 75

Wings, Whir of, 101
Wing-strokes, Phases of {see Pis.

iv.-vii.), 40-47, 84, 85

Rate of, 50, 99, 144

Short, 46

Unequal (PL ii), 30

Witherby, Mr. H. F., 66
Woodpecker, 55, 65
Weight, Mr. Wilbur and Mr.

Orville, 16, 153.

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The flight of birds. 1912/