Birds in Flight

by W. P. Pycraft

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The flight of birds has always aroused man’s envy and stirred his imagination. David longed for the wings of a dove: the writer of the Book of Proverbs tells us that “the way of an eagle” surpasses his understanding. Icarus, spurred on by dire necessity, actually, we are told, contrived to fly—but his maiden effort ended in disaster! To-day we have, in a sense, succeeded where he failed. But only because we have given up the idea of flight by personal effort, and make our aerial journeys in a flying machine.

That we owe much of our success to a study of the flight of birds is common knowledge, but the machine which has evolved as a consequence of this study pursues its way through the air after a very different fashion from that of the birds, for its vast body is thrust, or drawn, through the air by means of a propeller, driven at incredible speed, its immobile wings sustaining the weight. The wings of the bird, on the other hand, not only lift the body from the earth, but they sustain it in the air by their marvellously complex movements. And this is true, in varying degrees of bird, and bat, and butterfly: of dragon-fly and beetle.

Even they who must perforce dwell in crowded cities see daily the miracle of flight performed. For even here sparrows and pigeons, at least, are everywhere, and it is just because this is so, just because they have become so “common-place,” that their very presence escapes notice. Yet the wonder of their movements in the air might become a never-ending source of delight if only we went about our business with open eyes and minds alert.

Watch the wary sparrow spring from the ground and dart across the road, or up to the nearest house-top. How is it done with such incredible speed and accuracy?

To understand even the broad principles of flight, it is necessary to realize, at the very beginning, that the wing, in the case of the bird, or the bat, is a specially modified fore-leg. So also is the human arm and hand. But its transformation has not been so drastic as that of the bird, or the bat. Wherein the hand has been, as it were, completely re-modelled to fulfil the peculiar and complex functions demanded of it.

How should one describe the wing of a bird, as one sees it in flight?

The Dictionary, obscure and inaccurate as Dictionaries usually are, defines a wing as “the organ of a bird, or other animal, or insect, by which it flies—any side-piece.” Might not the impression one gathers of a wing, during flight, be defined as of a lateral extension of the body, presenting a relatively large surface, but having no appreciable thickness? That surface, examined in a dead bird, is seen to be formed, for the most part, of a series of parallel, tapering, elastic rods, fringed with an innumerable series of smaller, similar, but much shorter rods, closely packed, and linked together by some invisible means to form an elastic web? These we call the “quill,” or “flight-feathers.” The rest of the wing, and the body itself, is clothed with precisely similar structures, differing only in their smaller size. We call them “feathers” commonly, without realizing that they are the “Hall-mark” of the bird, for no other creature has ever been similarly clothed.

These quill-feathers play such a tremendously important part in flight that their arrangement, and relation to the underlying skeleton must be carefully examined by all who would understand the flight of birds. To begin with, then, note that they are so arranged as to overlap one another, the free edges of the quills facing the outer edge of the wing. Only by this arrangement would flight be possible, for on the upstroke of the wing through the air the quills act like the shutters of the sails of a windmill, allowing the wind to pass between them and so relieving pressure on the uplifting wing-stroke. On the down-stroke, the opposite effect is produced. The full force of the stroke is conserved, because, owing to the overlap, the several feathers are now pressed closely together to form an impervious sheet.