CHAPTER III.
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.
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."
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.
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.