Feet and Legs 389 
a web of skin which, when swimming, offers a large area 
of resistance to the water when the foot is pushed back- 
ward. The chick in the egg has a shadow-membrane 
of his fish-like ancestors between his toes, and in these 
water-birds the web of skin continues throughout life. In 
the terns or sea-swallows, which swim much less than 
they fly, the web is excised, or scalloped out deeply, a 
return to an almost semipalmated condition. 
Fic. 305.—Rough-legged Hawk in position of defence. 
A duck or swan out of sheer laziness will often hold 
one foot up out of the water and propel itself with the 
other, slightly altering the angle at which the web meets 
the water, so as to maintain a perfectly direct course. 
There is a little-known habit which I have frequently 
observed in captive ducks and several times in wild ones, 
of swimming thus with one foot when both eyes are shut 
and the bird is apparently fast asleep. But, in such a 
