The Sizes and Shapes of Wings and their relation to Flight.
"... the fowls of heaven have wings,
And blasts of heaven will aid their flight:
* * * * *
Chains tie us down by land and sea."--_Wordsworth._
The evasiveness of flight--The size of the wing in relation to
that of the body--Noisy flight--"Muffled" flight--The swoop of the
sparrow-hawk--The "flighting" of ducks--The autumn gatherings of
starlings and swallows--"Soaring" flights of storks and vultures--The
wonderful "sailing" feats of the albatross--The "soaring" of the
skylark--The "plunging" flight of the gannet, tern, and kingfisher.
Who needs to be told that birds fly? So
common-place has this fact become that the many, and varied forms of
wings, and the peculiarities of flight which are associated with these
differences, are rarely perceived. Even sculptors, and artists show a
hopeless unfamiliarity with the shapes of wings, and their meanings, at
any rate, as a general rule. Look at their attempts to display birds in
flight, or in the fanciful use of wings which convention has ascribed
to angels. For the most part these superbly beautiful appendages are
atrociously rendered.
Yet it must be confessed that any attempt to explain exactly how birds
fly must fail. We can do no more than state the more obvious factors
which are indispensable to flight, and the nature of its mechanism. The
subtleties, and delicate adjustments of actual flight evade us.
Our appreciation, however, of this supreme mode of locomotion will be
materially quickened, if we make a point of studying the varied forms
of flight as opportunities present themselves.
To begin with, it is worth noting that the size of the wing decreases
with the weight of the body to be lifted--up to a certain point, of
course. This, perhaps, may seem strange a statement to make. But it
can be readily verified. Compare, for example, the size of the body in
relation to the wings, in the case of the butterfly and the dragon-fly,
on the one hand, and the partridge and the crow, on the other. The two
first named, by comparison, have enormous wings.
Birds, it will be noticed, which haunt woods, or thickets, have short,
rounded wings, like the wren, the pheasant, or the tawny owl. Such, on
the other hand, as live in the open, like the gull, and the swallow,
have long, pointed wings. The reason for this is fairly plain. Birds
which must steer their course through the intricate mazes of a wood, or
thicket, would find their flight seriously hampered by long wings.
[Illustration: _Pheasants_]
These general principles once realized, a foundation is laid on which
one may base observations on the peculiarities of flight distinguishing
different types of birds.
Most of us, probably, at one time or another, in taking a walk through
the woods, have been startled, almost out of our wits, by a sudden
"whirr" of wings at our very feet; made by some crouching pheasant,
waiting till the very last moment before revealing himself, by taking
flight. This alarming noise is due to the shortness and stiffness of
the quill, or flight-feathers. With pinions moving with incredible
speed, the bird is off like a rocket. Not seldom, probably, it owes
its life to this ability to disconcert its enemies, till it has put a
safe distance between itself and danger. By way of contrast, let us
take the absolutely silent, easy movements of the owl, stealing forth
in the twilight of a summer's evening, seeking whom he may devour.
Here, again, we have a meaning in the mode of flight. Here silence is
more than golden: it means life itself. Nimble-footed, sharp-eared
mice and rats, must be snatched up before even the breath of suspicion
can reach them. The uncanny silence of this approach is rendered
possible, only by what may be called a "muffling" of the wings. For the
flight-feathers are not only of great breadth, but they are covered,
as it were, with velvet-pile, the "barbules" of the wing-quills, which
form the agents by which the "web" of the quill is held together,
having their upper spurs produced into long, thread-like processes,
which extinguishes any possibility of a warning "swish."
John Bright, in one of his magnificent perorations, caused his
spell-bound listeners to catch their breath, when, conjuring up a
vision of the Angel of Death, he remarked "we can almost hear the
rustle of his wings." One realizes the vividness of that imagery, when
one hears, as on rare occasions one may, the awe-inspiring rustle of
the death-dealing swoop of the falcon, or the sparrow-hawk, as he
strikes down his victim.
But the swish, and whistle of wings often stirs the blood with
delicious excitement, as, when one is out on some cold, dark night,
"flighting." That is to say, awaiting mallard passing overhead on the
way to their feeding ground, or in watching the hordes of starlings, or
swallows, settling down to roost in a reed-bed. No words can describe
these sounds, but those to whom they are familiar know well the thrill
of enjoyment they beget. There is no need, here, to muffle the sound of
the wing-beat. The falcon vies with the lightning in his speed, escape
is well nigh hopeless: neither have the swallows need for silence;
indeed, on these occasions, they add, to the music of their wings, the
enchantment of their twittering.
So much for flight in its more general aspects. Let us turn now to a
survey of some of the more remarkable forms of flight, beginning with
that known as "soaring."
This but few birds have mastered, and to-day it is rarely to be seen in
our islands, for eagles, falcons, and buzzards are, unfortunately, only
to be found in a few favoured localities. Happily, however, one may
yet realize the delight of watching a soaring buzzard, or raven, among
the hills of Westmorland, or in parts of Cornwall and Wales. But to
see the past-masters in the art, one must seek the haunts of pelicans,
vultures, and adjutant storks. The last-named is perhaps the finest
performer of them all. For the first hundred feet or so he rises by
rapid and powerful strokes of the wings, and then, apparently without
the slightest effort, or the suspicion of a wing-beat, he sweeps round
in great spirals, gaining some ten or twenty feet with each gyration,
the wings and tail all the while being fully extended and the primary
feathers widely separated at their tips. During the first part of
every turn he is flying slightly downward: at the end of the descent
he sweeps round and faces the wind, which carries him upward. Round,
round, he goes, mounting ever higher and higher, until at last he
attains a height of perhaps two miles.
The adjutant thus goes aloft apparently for the mere delight the
movement affords him. But not so with the vulture, who is a close rival
in this art. He soars for his very existence, for dead bodies are not
to be found everywhere. Possessing powers of sight infinitely greater
than ours, he mounts aloft for the purpose of taking observations. If
nothing "toothsome" can be seen from his vast range, he turns his
attention to the movements of such of his fellows as may be up on
the same errand miles away. Should he see one swooping earthwards he
instantly tracks him down, and is soon at the feast. This accounts for
the mysterious way in which vultures will gather together to the feast,
in a place where an hour ago not one was to be seen. A caravan of
camels, perchance, is making its toilsome way across a burning desert.
One falls by the way. In a few hours its bones will be picked clean by
a horde of these ravenous birds.
Longfellow sang the song of the vultures hunting in stately verse:--
"Never stoops the soaring vulture
On his quarry in the desert,
On the sick or wounded bison,
But another vulture, watching
From his high aerial look-out,
Sees the downward plunge and follows,
And a third pursues the second,
Coming from the invisible ether,
First a speck, and then a vulture,
Till the air is thick with pinions."
[Illustration: Black-game.]
Darwin, in his wonderful "Journal of a Voyage Round the World" gives a
marvellously vivid word-picture of the largest, and most interesting of
all the vultures, the Condor of the Andes--one of the largest of flying
birds, having a wing-span of something over nine feet:--
"When the condors are wheeling in a flock round and round any spot,
their flight is beautiful. Except when rising from the ground, I do
not recollect ever having seen one of these birds flap its wings.
Near Lima, I watched several for nearly half an hour, without once
taking off my eyes; they moved in large curves, sweeping in circles,
descending and ascending without giving a single flap. As they glided
close over my head, I intently watched, from an oblique position, the
outlines of the separate and great terminal feathers of each wing;
and these separate feathers, if there had been the least vibratory
movement, would have appeared as if blended together; but they were
seen distinctly against the blue sky. The head and neck were moved
frequently, and, apparently, with force, and the extended wings seemed
to form the fulcrum on which the movements of the neck, body, and the
tail acted. If the bird wished to descend, the wings for a moment
collapsed; and then again expanded with an altered inclination, the
momentum gained by the rapid descent seemed to urge the bird upwards
with the even and steady movement of a paper kite. In the case of
any bird _soaring_, its motion must be sufficiently rapid, so that
the action of the inclined surface of its body on the atmosphere may
counterbalance its gravity. The force to keep up the momentum of a
body moving in a horizontal plane in the air (in which there is so
little friction) cannot be great, and this force is all that is wanted.
The movement of the neck and body of the condor, we must suppose, is
sufficient for this. However this may be, it is truly wonderful and
beautiful to see so great a bird, hour after hour, without apparent
exertion, wheeling and gliding over mountain and river."
Those who "go down to the sea in ships" have to face many perils, but
the "wonders of the great deep" are for them a lure. One of these is
to watch the marvellous "sailing" flights of the wandering albatross.
His wings have, when expanded, a peculiarly "ribbon-like" form,
and measure from tip to tip, over eleven feet--thus exceeding that
of the condor, which, however, is the heavier bird of the two. The
"ribbon-like" form of the wings is due to the extreme shortness of the
flight-quills--the primaries and secondaries, and the great length of
the arm and fore-arm. And it may be to these structural peculiarities
that the "sailing" flight just alluded to is due. Resembling soaring in
many of its aspects, yet it differs materially in that it is performed
low down, not at immense heights. The most graphic description of
these movements is surely that of Mr. Froude: "The albatross," he
tells us, "wheels in circles round and round, and for ever round the
ship--now far behind, now sweeping past in a long rapid curve, like a
perfect skater on a perfect field of ice. There is no effort; watch
as closely as you will, you will rarely see, or never see, a stroke
of the mighty pinion. The flight is generally near the water, often
close to it. You lose sight of the bird as he disappears in the hollow
between the waves, and catch him again as he rises over the crest;
but how he rises, and whence comes the propelling force, is, to the
eye, inexplicable; he alters merely the angle at which the wings are
inclined; usually they are parallel to the water and horizontal; but
when he turns to ascend, or makes a change in his direction, the wings
then point at an angle, one to the sky, the other to the water."
One sometimes hears the skylark described as "soaring" upwards, when
performing that wonderful musical ride which has made him so famous.
But as, spell-bound, one listens to his rapturous strains, and watches
his spiral ascent, one cannot help noticing that his wings are never
still, they seem almost to be "beating time" to his music. In true
soaring they are scarcely ever moved.
The upward progress of a bird when soaring is, of necessity,
comparatively slow. But in what we may call "plunging" flight the case
is very different, for here the velocity of the descent is great.
The frigate-birds of tropical seas, and the gannet of our own, display
this mode of flight to perfection. It is worth going far to see a
gannet dive. Travelling at a relatively considerable height, and
eagerly scanning the surface of the water for signs of a shoal of
fish, this amazing bird dives with the speed of lightning, and with
half-spread wings disappears with a terrific plunge beneath the
surface, to emerge, an instant later, with his prey. One can measure
the force of such a plunge by the cruel trick, sometimes played by
fishermen, of fastening a herring to a board, and setting it adrift
where gannets are about. The unsuspecting victim descends as usual
upon his prey, only to meet instant death by the shock of his impact
with the board. Those who talk glibly of identifying birds by their
flight may point to this wonderful diver as a case in point. But while
one may often see the gannet on the wing, it is by no means so often
that one will have the good fortune to see him dive, for he is not
always hungry. His white body, pointed tail, and black quill-feathers
would then enable the novice to name him at once. But--in his immature
plumage, he would, at a little distance, appear black, and unless he
were fishing, the chances of recognition would be by no means great.
Close at hand he would appear speckled with white.
[Illustration: _Brown Owl_]
But this by the way. There are two other birds which dive from a
height on the wing. One of these is the kingfisher: the other is the
tern. The term "tern" is here used collectively, for there are several
species, but all have this habit of diving from a height. During the
summer months one may be quite sure of an opportunity of watching the
graceful, easy flight of at least three species. For they haunt the
sea-shore, river, and lake with equal impartiality. Those who are on
the look-out for terns, for the first time, will easily recognise
them. For, in the first place they look like miniature gulls, but
with longer and more pointed wings, and forked tails. Further, all
have a characteristic black cap. They travel in small parties, as if
for company, keeping no more than a yard or two from the surface of
the water, and scanning it eagerly in search of shoals of small fish,
or crustacea. As these are found one will note a quickening of the
wing-beat, and a sudden dive, like that of the gannet, with half-closed
wings. And sometimes, too, the impetus will take them completely under
water.
[Illustration:
1 Bat
2 Butterfly
3 Beetle
4 Dragon-Fly
5 Bone of Birds Wing, Showing the three Divisions,
Arm--Fore-arm--Hand.
6 Breast Bone of Swan
7 " " " Pigeon
8 " " " Pelican
9 & 10 Apteryx, Cassowary (degenerate wings).
]