144 THE FLIGHT OF BIRDS 
unable to get the wind to lift them. And yet, rela- 
tively to their bulk an#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 
