STABILITY 33 
more so. But the area varies so much, according 
as they are expanded or not, that I have found 
it very difficult to give measurements. However, 
we can bear in mind that the tail of the Sparrow- 
Hawk, for example, is not only very long but very 
broad, whereas the tail of the Duck is not only short, 
but narrow and weak, its inefficiency being even 
more marked than its small size. 
It is noteworthy that the bird which stands 
at the head of the list as being remarkable for size 
of tail is not a particularly short-legged bird. Two 
cock Sparrow-Hawks, of which I took measurements, 
had each legs 7 inches long (the toes being included). 
However, the legs are very thin, as if not intended 
for standing, but, armed as they are with long, 
efficient claws, for seizing a victim. It is often 
maintained that Hawks hang their legs down during 
flight, but this is certainly not usually the case. 
However, they could on occasion be lowered, to give 
the tail freer play, without much affecting the bird’s 
equilibrium. 
The main function of the tail is to prevent loss 
of equilibrium, and when large it plays its part 
wonderfully well. In spite of its great expanse, 
its weight is a negligible quantity and its working 
is splendidly prompt. Evolution has done wonders 
in thus metamorphosing the long, heavy tail of the 
bird’s reptilian ancestor. A number of vertebrae 
have been compressed to form the pygostyle, the 
small bony base of this wonderful piece of machinery. 
What weight there is is almost all accounted for 
by this little bone and the muscles that move the 
large spread of feathers. The muscles have the 
D 
