Birds in Flight

by W. P. Pycraft

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.

Wild Duck