Fig. 104.—The Cape Barn-Owl (Strix capensis, Smith), as seen in full flight, hunting. The under surface of the wings and body are inclined slightly upwards, and act upon the air after the manner of a kite. (Compare with fig. [59], p. 126, and fig. [102], p. 183.)—Original.
The albatross (fig. [105], p. 200) and pelican afford instances of the third class, embracing the heavy-bodied, long-winged birds.
The duck (fig. [107], p. 204), pigeon (fig. [106], p. 203), crow and thrush, are intermediate, both as regards the size of the wing and the rapidity with which it is made to oscillate. These constitute the fourth class.
The albatross (fig. [105], p. 200), swallow, eagle, and hawk, provide instances of sailing or gliding birds, where the wing is ample, elongated, and more or less pointed, and where advantage is taken of the weight of the body and the shape of the pinion to utilize the air as a supporting medium. In these the pinion acts as a long lever,[97] and is wielded with great precision and power, particularly at the shoulder.
The Flight of the Albatross compared to the Movements of a Compass set upon Gimbals.—A careful examination of the movements in skimming birds has led me to conclude that by a judicious twisting or screw-like action of the wings at the shoulder, in which the pinions are alternately advanced towards and withdrawn from the head in a manner analogous to what occurs at the loins in skating without lifting the feet, birds of this order can not only maintain the motion which they secure by a few energetic flappings, but, if necessary, actually increase it, and that without either bending the wing or beating the air.
The forward and backward screwing action of the pinion referred to, in no way interferes, I may remark, with the rotation of the wing on its long axis, the pinion being advanced and screwed down upon the wind, and retracted and unscrewed alternately. As the movements described enable the sailing bird to tilt its body from before backwards, or the converse, and from side to side or laterally, it may be represented as oscillating on one of two centres, as shown at fig. 105; the one corresponding with the long axis of the body (fig. 105, a b), the other with the long axis of the wings (c d). Between these two extremes every variety of sailing and gliding motion which is possible in the mariner’s compass when set upon gimbals may be performed; so that a skimming or sailing bird may be said to possess perfect command over itself and over the element in which it moves.
