Animal Locomotion; or, walking, swimming, and flying / With a dissertation on aëronautics

by James Bell Pettigrew

Fig. 102.—Shows the upward inclination of the body and the flexed condition of the wings (a b, e f; a´ b´, e´ f´) in the flight of the kingfisher. The body and wings when taken together form a kite. Compare with fig. [59], p. 126, where the wings are fully extended.

Flexion of the Wing necessary to the Flight of Birds.—Considerable diversity of opinion exists as to whether birds do or do not flex their wings in flight. The discrepancy is owing to the great difficulty experienced in analysing animal movements, particularly when, as in the case of the wings, they are consecutive and rapid. My own opinion is, that the wings are flexed in flight, but that all wings are not flexed to the same extent, and that what holds true of one wing does not necessarily hold true of another. To see the flexing of the wing properly, the observer should be either immediately above the bird or directly beneath it. If the bird be diminishing from before, behind, or from the side, the up and down strokes of the pinion distract the attention and complicate the movement to such an extent as to render the observation of little value. In watching rooks proceeding leisurely against a slight breeze, I have over and over again satisfied myself that the wings are flexed during the up stroke, the mere extension and flexion, with very little of a down stroke, in such instances sufficing for propulsion. I have also observed it in the pigeon in full flight, and likewise in the starling, sparrow, and kingfisher (fig. 102, p. 183).

It occurs principally at the wrist-joint, and gives to the wing the peculiar quiver or tremor so apparent in rapid flight, and in young birds at feeding-time. The object to be attained is manifest. By the flexing of the wing in flight, the “remiges,” or rowing feathers, are opened up or thrown out of position, and the air permitted to escape—advantage being thus taken of the peculiar action of the individual feathers and the higher degree of differentiation perceptible in the wing of the bird as compared with that of the bat and insect.

In order to corroborate the above opinion, I extended the wings of several birds as in rapid flight, and fixed them in the outspread position by lashing them to light unyielding reeds. In these experiments the shoulder and elbow-joints were left quite free—the wrist or carpal and the metacarpal joints only being bound. I took care, moreover, to interfere as little as possible with the action of the elastic ligament or alar membrane which, in ordinary circumstances, recovers or flexes the wing, the reeds being attached for the most part to the primary and secondary feathers. When the wings of a pigeon were so tied up, the bird could not rise, although it made vigorous efforts to do so. When dropped from the hand, it fell violently upon the ground, notwithstanding the strenuous exertions which it made with its pinions to save itself. When thrown into the air, it fluttered energetically in its endeavours to reach the dove-cot, which was close at hand; in every instance, however, it fell, more or less heavily, the distance attained varying with the altitude to which it was projected.

Thinking that probably the novelty of the situation and the strangeness of the appliances confused the bird, I allowed it to walk about and to rest without removing the reeds. I repeated the experiment at intervals, but with no better results. The same phenomena, I may remark, were witnessed in the sparrow; so that I think there can be no doubt that a certain degree of flexion in the wings is indispensable to the flight of all birds—the amount varying according to the length and form of the pinions, and being greatest in the short broad-winged birds, as the partridge and kingfisher, less in those whose wings are moderately long and narrow, as the gulls, and many of the oceanic birds, and least in the heavy-bodied long and narrow-winged sailing or gliding birds, the best example of which is the albatross. The degree of flexion, moreover, varies according as the bird is rising, falling, or progressing in a horizontal direction, it being greatest in the two former, and least in the latter.

It is true that in insects, unless perhaps in those which fold or close the wing during repose, no flexion of the pinion takes place in flight; but this is no argument against this mode of diminishing the wing-area during the up stroke where the joints exist; and it is more than probable that when joints are present they are added to augment the power of the wing during its active state, i.e. during flight, quite as much as to assist in arranging the pinion on the back or side of the body when the wing is passive and the animal is reposing. The flexion of the wing is most obvious when the bird is exerting itself, and may be detected in birds which skim or glide when they are rising, or when they are vigorously flapping their wings to secure the impetus necessary to the gliding movement. It is less marked at the elbow-joint than at the wrist; and it may be stated generally that, as flexion decreases, the twisting flail-like movement of the wing at the shoulder increases, and vice versâ,—the great difference between sailing birds and those which do not sail amounting to this, that in the sailing birds the wing is worked from the shoulder by being alternately rolled on and off the wind, as in insects; whereas, in birds which do not glide, the spiral movement travels along the arm as in bats, and manifests itself during flexion and extension in the bending of the joints and in the rotation of the bones of the wing on their axes. The spiral conformation of the pinions, to which allusion has been so frequently made, is best seen in the heavy-bodied birds, as the turkey, capercailzie, pheasant, and partridge; and here also the concavo-convex form of the wing is most perceptible. In the light-bodied, ample-winged birds, the amount of twisting is diminished, and, as a result, the wing is more or less flattened, as in the sea-gull (fig. 103).

Fig. 103.—Shows the twisted levers or screws formed by the wings of the gull. Compare with fig. [53], p. 107; with figs. [76], 77, and 78, p. 147, and with figs. [82] and 83, p. 158.—-Original.