144 THE FLIGHT OF BIRDS 



unable to get the wind to lift them. And yet, rela- 

 tively to their bulk and weight, their wings are very 

 decidedly larger in area than those of big birds. The 

 explanation is, I believe, that, though the wing of 

 the small bird is relatively the larger, yet, actually, 

 it presents too small an expanse for the purpose of 

 soaring. The wind, instead of giving the required 

 support, escapes at the edges. In ordinary flight, 

 when the beating wings move with enormous 

 rapidity, their small area does not tell against them 

 as it does when the bird merely remains passive and 

 waits for the wind to strike it. In fact the velocity 

 of the wing's movement during the down-stroke is 

 distinctly greater than the velocity of the wind that 

 supports the soaring bird. Even a Heron or a Crow 

 in leisurely flight takes not less than 120 strokes 

 per minute, the Pigeon, according to Professor Marey, 

 480, the Duck 540, and the Sparrow 780. This 

 means that the farther part of the wing moves with 

 astonishing rapidity. The big soaring bird, more- 

 over, has a large cup-like concavity near the base of 

 the wing, which must hold the wind and so give 

 much support. Even the Gannet's wings, narrow 

 and elegant as they are, have near the body a deep 

 hollow that serves to catch and utilise any up-current 

 that offers. 



Soaring always goes on at a considerable altitude. 

 In mountainous countries there are frequent oppor- 

 tunities of seeing it, and with luck one may occa- 

 sionally get near to the scene of the performance. 

 In Spain I once climbed to the top of a high cliff on 

 which was a Vulture rookery, some fifteen nests of 

 the Griffon Vulture, and saw the great birds circling 



