30 THE FLIGHT OF BIRDS 
we are not conscious@ any difference between the 
voluntary and the involuntary. In the case of a 
bird the line is still harder to draw, not only because 
we cannot get within his mind, but because the 
movements which we are bound to label voluntary, 
since they cannot be mere unconscious reflexes, are 
so largely instinctive. When the young Swallow 
takes the first plunge from the parental nest and 
trusts himself to the air, he finds at once that he 
can fly ; the power of flight is instinctive. A child 
has to learn by much practice to co-ordinate his 
muscles; no other young creature is so devoid of 
instinctive skill. The young Swallow, though he 
can co-ordinate his muscular activities well enough 
to fly with some success, has, of course, much to 
learn. At the outset he misses most of the flies and 
gnats, and his parents have to come alongside and 
put their captures in his mouth. But he seems by 
instinct to spread his tail when it should be spread, 
and, no doubt, though it is hard to see this, he takes 
a harder stroke with one wing than the other, when 
a harder stroke is required. For fore-and-aft balance 
he depends largely on his tail. He has not a long 
neck that he can bend or straighten out, and his legs 
are so short and light that no movement of them can 
have much effect. For lateral stability he must, as 
far as voluntary adjustments are needed, depend 
mainly on unequal wing-strokes.* That birds do 
take unequal strokes has been clearly proved by the 
camera in the case of Pigeons, Owls, Gannets and 
some others (see Pl. 1). This inequality, sometimes 
so conspicuous in a photograph, the human eye has 
* See Chap. V. 
