Modes of Flight.
"The soaring lark is blest as proud
When at Heaven's gate she sings:
The roving bee proclaims aloud
Her flight by vocal wings."--_Wordsworth._
The movements of the wing in flight--Marey's experiments--Stopping
and turning movements--Alighting--"Taking off"--Hovering--The use of
the tail in flight--The carriage of the neck in flight--And of the
legs--The flight of petrels--The speed of flight--The height at which
birds fly--Flight with burdens--Experiments on the sizes of the wing
in relation to flight--Flight in "troops."
While it is possible to show that certain
kinds of flight are to be associated with such and such peculiarities
of the skeleton, and the muscles attached thereto, there are many
"eccentricities" which cannot be measured, and explained, in terms of
mechanism.
The very disconcerting, twisting, flight of the snipe is one of these.
The sportsman knows it well: and he knows that the twisting, during
which the bird turns the body half over--that is with, say, the
left wing pointing directly downwards, and the right wing directly
upwards--is only the preliminary to getting fully on the way, and that,
presently, it will pursue a straight course, with arrow-like speed.
Yet its cousin, the jack-snipe, never twists.
Why does the woodcock invariably drop after a charge of shot, even
though not a pellet has touched it, while a snipe pursues its way?
These differences are not merely differences of "habit": they indicate
subtle differences in nervous response to the same kind of stimulus,
and in structural details yet to be unravelled.
Some day the cinematograph will reveal to us all the phases of flight
and the movements to which they are due. Even now, thanks to the modern
camera, we have learned a great deal. We have learned, for example,
that the flight of a bird is not effected merely by rapid up and down
movements of the fully extended wings, or with flexed wings--that is
to say, half closed, as in "gliding" flight when a bird is descending,
or in the swoop of, say, the sparrow-hawk. Only in one of these two
positions do we ever seem to see the wings when we have to trust to our
eyes alone, as the bird hurries past us. The impression that we have
seen aright is confirmed when we stand on the deck of a steamer, and
watch the gulls following in its wake. For incredibly long distances
they will travel without a perceptible wing-beat. The albatross is the
finest of all performers in regard to this kind of flight, which is
due, apparently, to air currents created by stiff breezes, or gales.
Some birds seem to make their way against a head-wind with the minimum
of effort, by partly flexing the wings and gliding downwards: at the
end of the descent, by turning the body sharply upwards, and spreading
the wings to the fullest extent, they are lifted up, and driven
forward, like a kite.
Marey and Pettigrew, long ago, showed conclusively, by means of
photography, that our conception of the movement of the wing during
flight was far from correct.
To avoid a long and tedious description, and many technicalities, it
must suffice to say that the wing of a bird possesses very considerable
freedom and range of movement at the shoulder joint. Certainly, during
some phases of flight, the wings are thrust forward and extended to
their fullest extent, so that the outer margins of the wings come to
lie almost parallel with the long axis of the body, as may be seen in
the spirited illustration showing the goshawk in flight. As they sweep
downwards, and backwards, they lift the body and drive it forwards.
At the end of the "sweep" they are "flexed," that is to say, bent at
the elbow and wrist-joints, while at the same time they are raised and
brought forward above the body for a repetition of the stroke. These
movements are too quick for the eye to follow, but they have been fixed
for us by the camera.
Marey devised an ingenious experiment in his endeavour to discover the
movements of the bird's wing during flight. He fastened a small piece
of paper to the tip of a crows wing, and as the bird flew in front of
a perfectly black screen he took a photograph of this moving speck of
white, while, of course, no image of the crow appeared on the plate.
The resultant picture gave a series of "figure of 8 loops" as one would
make this figure with a pen, contriving to make the lower loop very
small, and the upper loop very large. But as the wing-beat increased in
speed the lower loop gradually faded out.
These movements of the wing, however, are descriptive rather of what
takes place during very vigorous flight, as when the bird is getting
up "steam." When he is well under way there is no need for these long
and very tiring strokes, except in the case of birds like the pheasant
or the duck. A gull, when in full career does not, apparently, raise
the wings very high, nor depresses them very low, nor does it flex the
wings at the wrist-joints.
Stopping and turning movements are generally extremely difficult to
follow, because they are performed so quickly. They can be seen fairly
easily in the case of some of the larger birds. Ducks, as is well shown
in one of our coloured Plates, draw the head backwards, tilt the body
upward, thrust the feet forward, and spread the tail, at the same time
turning it forwards. Gulls and pigeons too may be watched with profit.
[Illustration: _Wild Duck_]
In turning, the body is tilted sideways, so that the tip of one wing
points skywards, the other earthwards, as in the case of the goshawk
illustrated in this book. The pigeon, and some other birds seem further
to spread out the long, stiff quills borne by the thumb, which form
what is known as the "bastard-wing." This turning movement is well
shown, again, in the very realistic coloured picture of the woodcock
turning in mid-air, and bearing too the burden of one of its nestlings.
If it is difficult to satisfy oneself as to the way in which a bird
alights, it is no less so to detect its movements in taking wing.
Most of us must have seen sparrows making this effort from the road,
thousands of times. But ask of anyone, How is it done? The act takes
place so quickly that the eye cannot follow its execution. And what is
true of the sparrow is true of most birds. But there are some where
this is not the case. Many water-birds, the cormorant, for example,
get under way but slowly, and with evident effort. They flap along
the surface for some distance before they gain sufficient impetus to
lift them into the air. And there are many long-winged, short-legged
birds which can rise from a level surface only with great difficulty,
or not at all. The swift is one of these, for its legs are excessively
short. The albatross is another: and this is true, indeed, of many of
the petrel-tribe. The puffin, again, seems unable to rise on the wing
from the ground. It appears invariably to run along until it reaches
the edge of cliff which lodges its burrow, and then, as it were,
throw itself over the edge. The heron, when springing into the air,
stretches his long neck out to its fullest extent, and presents a pair
of dangling legs, well shown in one of our coloured Plates, but when
once fully on the way its pose entirely changes, the neck being drawn
in and the legs thrust out backwards.
Flight does not always mean progress through the air. Most birds can,
at need, arrest their course, and hang, as it were, suspended in the
air. In the beautiful coloured plate, representing the chaffinch
hovering over its half-fledged young, and in that of the kingfisher
and its young, this form of "hovering" flight can be seen. But the
greatest of all exponents in the art of hovering is the kestrel, known
also, for this very reason, as the "windhover." It is most fascinating
to watch this bird hang, as it were, from the clouds, motionless,
yet with quivering wings, as he scans the ground below in his search
for some unsuspecting mouse. It is hard, indeed, to say which is the
more wonderful, this power of remaining stationary for comparatively
long periods in the air, or the surprising powers of sight which this
bird possesses. During these hovering movements, always head to wind,
it will be noted, the tail plays a very important part, being spread
to its extremest limit, and at the same time thrust forward beneath
the body. In some birds this forward movement is more marked than in
others. And this because such birds possess a somewhat more flexible
spine, there being a certain amount of "play" where the vertebræ of the
loins join the welded mass of vertebræ which lie between the bones of
the hip-girdle.
But the tail feathers are not indispensable. This much is shown in the
case of birds like the kingfisher, the water-hen, and the land-rail,
which contrive to fly well, and at a great pace, though they have but
the merest apology for a tail. More than this, the grebes have no
tail at all. But it is to be noted that they are by no means adept at
turning movements; owing to the lack of this appendage the body, when
in mid-air, has a curiously truncated appearance, as may be seen in
the illustration. Further, it is significant that in the contemptible
"sport" of pigeon-shooting from traps, the birds are deprived of their
tails to prevent them from making turning movements.
The carriage of the head and neck, and of the legs, during flight
presents some interesting, and some instructing contrasts.
Ducks, geese, and swans, flamingoes, storks, and cormorants always fly
with the head and neck stretched out to their fullest extent. Herons
and pelicans, though also long-necked birds, draw the head back till it
rests almost on the shoulders. Most birds, indeed, fly with the head
drawn back towards the body. The appearance of some of these birds on
the wing can be seen at a glance on turning to the page illustrating
this aspect of flight.
Not so very long ago a great controversy was waged as to what birds did
with their legs during flight. Many of the older artists invariably
depicted them drawn up under the breast. But as a matter of fact,
this method seems to be confined to the Passerine birds--the "perching
birds," such as crows and finches and their kin. It has yet to be
settled what obtains among what are known as the "Picarian" birds, such
as kingfishers, bee-eaters, woodpeckers, and so on. The legs and feet
of these birds are so small, and their flight is so rapid, that the
matter is by no means an easy one to settle. But all other birds carry
the legs and toes bent backwards, under the tail. In the gulls, this
can easily be seen, and easier still in the case of the common heron,
where they are, as it were, trailed out behind--owing to the shortness
of the tail and the great length of the leg. The puffin carries them
"splayed" out on each side of his tail, and so also do his kinsmen, the
razor-bills, and guillemots.
The legs, as a rule, take no part in flight. True, they can be seen
thrust out just before alighting, but this is solely for the purpose
of effecting a safe landing. But where gulls can be watched at close
quarters, as in harbours, round a ship, or in such favoured spots as
are to be found about the bridges of London during the winter, careful
watch will show that the legs are frequently used when efforts are
being made to turn, or check the speed of flight.
Some of the smaller petrels--like the storm-petrel, or "Mother Carey's
chickens," will patter over the water with their feet as they fly just
over the surface of the waves.
[Illustration: _Sketches of Ducks in flight 1922_
1. }
} Scaup.
1a. }
2. Goldeneye.
3. }
} Pochard.
4. }
5. }
to } Mallard.
10. }
]
Whether the legs are carried drawn close up beneath the breast, or
thrust backwards under the tail, the purpose of this disposal is the
same--to prevent any interference with the "stream-lines" of the body
which would impede flight.
On the matter of the speed of flight there seems to be much
misconception. Gätke, the German ornithologist, gravely asserted that
the little Arctic blue-throat--one of our rarer British birds--could
leave its winter resort in Africa in the dusk of evening, and arrive at
Heligoland--where he spent so many years studying bird migration--nine
hours later. That is to say it could travel 1,600 geographical miles
in a single night, at the astounding velocity of 180 miles an hour!
According to another estimate of his, curlews, godwits, and plovers
crossed from Heligoland to the oyster-beds lying to the eastward, a
known distance of rather more than four English miles, in one minute;
or at the rate of over 240 miles an hour. Against such extravagant
estimates it is hardly necessary to bring rebutting evidence. But if
any be demanded it may be furnished by the carrier pigeon, which has
been known to maintain a speed of 55 miles an hour for four hours in
succession: and it is extremely unlikely that this is much, if at all,
exceeded by any wild bird during long-distance flights.
That our spring and autumn migrants must possess wonderful powers of
endurance is beyond question. And it is equally certain that thousands
must perish by the way. By this means is the standard of flight
maintained--the weak perish. Even the minimum standard of efficiency
for the survival of such an ordeal must be a high one.
Few of us see anything of these marvellous migration flights. For, in
the first place, they are generally performed at night, and at a great
height, often beyond the range of human vision. Only as they approach
land, and their destination, do they descend. American naturalists
have made some interesting observations by directing a telescope
against the sky. Thus, Mr. Frank Chapman, by turning his instrument
towards the full moon, has seen birds passing at night at an altitude,
according to his computation, of five miles: while the late Mr. W.
E. D. Scott saw, through an astronomical telescope at Princeton,
New Jersey, great numbers of birds passing across the face of the
moon--warblers, finches, and woodpeckers among them. Mr. Chapman again,
on another occasion, saw no less than 262 birds pass over the field of
his telescope at a height of from 1,500 to 15,000 feet: and the most
remarkable thing of all was the fact that the lowest birds were flying
upwards, as if they had risen from the immediate neighbourhood and were
seeking the proper elevation to continue their flight.
As has already been remarked, when nearing their destination migrating
birds descend, though still many miles from land. Should a gale be
raging they fly so low that they barely top the waves. And this,
apparently, to escape, so far as is possible, the force of the wind.
Larks, starlings, thrushes, and other small birds, can sometimes be
seen during daylight crossing the North Sea in their thousands. At such
times many will often afford themselves a brief rest in the rigging
of ships, homeward bound, but the main host hurry on. The beautiful
golden crested wren, our smallest British migrant, is one of these. A
glance at our charming coloured plate will show at once that the wing
is not that of a bird of strong flight. There is no more interesting
experience to the bird-lover than that of watching the tired travellers
drop earthwards, as they leave the dreadful sea behind them.
With all birds yet retaining the power of flight there is always a
liberal "margin of safety" in regard to the wing area. That is to
say this is always in excess of the minimum area necessary to make
flight possible. This much, indeed, is manifest from the fact that the
eagle can bear off a victim equalling himself in weight. Should he
miscalculate, he can always drop his burden, or lessen its weight by
eating part of it on the spot. Not so the osprey, or the sea-eagle,
which have been known to plunge down and drive their talons into
fishes too large to be raised. Unable to release their grip, death, by
drowning, has inevitably followed.
Sometimes the burden is a passenger, instead of a victim. One of
the most striking of the coloured plates in this volume is that of a
woodcock carrying one of its nestlings to a distant feeding place. This
habit is well known. It is not often that the necessity arises, but
there are occasions where suitable nesting and feeding grounds cannot
be found together, or when, as during prolonged drought, the normal
feeding area dries up. Then, instinctively, the parent will surmount
the dangers of starvation for their offspring, by conveying them to a
land of plenty, returning again to the shelter of the wood as soon as
the meal is over. The weight of a newly-hatched nestling, it is true,
could scarcely be called a "burden." But they are carried about thus
until they are strong enough to perform the journey for themselves.
Thus, then, towards the end of the nursing period the weight to be
carried is by no means a light one.
But it was shown, long since, by direct experiment, that the area of
a bird's wing is considerably in excess of what is required for the
purpose of flight. Dr. J. Bell Pettigrew, more than fifty years ago,
to test this matter, cut off more than half of the secondary wing
feathers of a sparrow, parallel with the long axis of the wing. He
first clipped one, then both wings, and found that in both cases flight
was apparently unimpaired. He then removed a fourth of the primary
feathers--the outermost quills--and still the flight was unimpaired. At
any rate the bird flew upwards of thirty yards, rose to a considerable
height and alighted in a tree. Thirty yards, however, is a short
flight even for a sparrow. But it is enough to show that flight, if not
_sustained_ flight, was possible after this mutilation. Not until more
than one-third of the quills along the whole length of the wing were
removed, did the flight become obviously laboured. And he found that
what was true of the sparrow, was equally true of the wings of insects.
Though these experiments demonstrate, in a very unmistakable manner,
that flight with a greatly reduced wing area is possible, we have no
evidence that this reduction would make no difference to the length of
time the bird could remain on the wing. And this is a very important
matter.
An aspect of flight which has now to be considered is that of birds
which fly in troops. Some species always travel thus, others only on
occasions. Rooks and gulls afford instances of this, when, during windy
weather, or for other reasons, they congregate and fly round and round
in great circles, at a considerable height. Small wading-birds, like
ringed plovers and dunlin, commonly fly in "bunches." The last named
furnish a singularly interesting sight when thus travelling; for their
evolutions are so amazingly timed. As if at a given signal every bird
in the troop will change its course at the same moment, and in the
same direction, so that now one sees a flickering mesh-work of grey,
and now a shimmering as of snow-flakes, as first the grey backs, and
then the white breasts are turned towards one. But flights such as this
are to be seen only during the autumn and winter months. For during
the breeding season these little flocks are broken up and distributed
far and wide. But there is yet another reason. They wear a totally
different dress--the courtship or breeding plumage. Herein the upper
parts are of a rich chestnut hue, streaked with black, while the under
parts are black. Even more fascinating to watch are the autumn troops
of starlings on the way to their roosting places. Hundreds at a time,
not to say thousands, take part in these flights. Now they rush onward,
in one great far-flung sheet, and now they close up into a great,
almost ball-like, mass: and now they thin out till they look like a
trail of smoke. But always they wheel and turn and rise and descend,
not as separate bodies, but as one. How are such wonderful evolutions
timed. The movements of an army on review-day are not more precise,
or more perfectly carried out. During the whole flight not a sound,
save the swishing of their wings can be heard. The marvel of it all is
beyond the range of words, nor can one express the peculiar delight
such a sight affords.
Why is it that ducks and geese commonly fly either in Indian file, or
in a roughly V-shaped formation, with the apex of the V forward? Why do
they not fly all abreast? One cannot say, but they never do.
Some mention must be made here of the surprising numbers in which
geese, of some species, congregate. Writing of the Brent goose, in his
"Bird Life of the Borders," Mr. Abel Chapman--and there are few men who
can write with such authority on the subject--tells us:--"Just at dark
the whole host rise on the wing together, and make for the open sea. In
the morning they have come in by companies and battalions, but at night
they go out in one solid army; and a fine sight it is to witness their
departure. The whole host, perhaps ten thousand strong, here massed in
dense phalanxes, elsewhere in columns tailing off into long skeins, V's
or rectilineal formations of every conceivable shape, (but always with
a certain formation)--out they go, full one hundred yards high, while
their loud clanging, defiance--"honk, honk,--torrock, torrock," and its
running accompaniment of lower croaks and shrill bi-tones, resounds for
miles around."
[Illustration: Peregrine chasing Duck.]