98 THE FLIGHT OF BIRDS 
comparatively big birdggshe Grouse and the Partridge. 
The great concavities beneath their wings, no doubt, 
aid them much. But the intervals of gliding, 
depending as they do on the preliminary attainment 
of great momentum, necessarily come at rarer 
intervals than they do in the case of the small bird. 
I am not speaking here of the long glides achieved 
without loss of altitude by some large, or fairly large, 
birds when the wind has an upward trend. That is 
a feat of the same nature as soaring, and I reserve 
it for another chapter. 
In another way the big and the small bird present 
a striking contrast. The former is often the slave 
of his own momentum, and only with difficulty and 
effort deviates from his line of advance ; the Goose 
and the Duck are good examples of this. The 
Swallow, on the other hand, turns with the utmost 
agility and ease. Some bigger birds have a wonder- 
ful nimbleness, though, in this respect, I think, they 
are no match for the Swallow. The Sparrow-Hawk, 
as he pursues some coveted small bird, turns and 
twists among the trees and bushes with great skill, 
his long, broad tail helping him very much in steering. 
Birds of prey must, of course, have great nimbleness, 
great power of suddenly checking themselves or sud- 
denly changing their course ; otherwise they would 
be unsuccessful as hunters, and would dash themselves 
to pieces when swooping upon some intended victim 
that was flying not far above the ground or a tree. 
The Lapwing turns to account his power of putting 
on the brake and making sudden turns by dashing 
at his enemy, alarming him, then suddenly pulling 
up and retreating only to make another dash. The 
