CLASS AVES 



413 



tion. Steering and balancing are done by adjustments of the tail and 

 wings. The feather covering insulates the body perfectly, prevents loss 

 of heat and consequent chilling, and enables the bird to maintain under 

 all conditions a practically constant temperature. In order to secure 

 heat regulation and at the same time to guard against too high a tempera- 



Feet of birds. From mounted specimens. Showing the inner side of the 



foot, except J. A, wading foot of greater yellowlegs. X 3^. B, totipalmate foot of 

 cormorant. X /i- C, swimming foot of blue-winged teal duck. X x'S- D, generalized 

 foot of ring-necked pheasant. X %. E, perching foot of yellowthroat. About natural 

 size. F, raptorial foot of Swainson hawk 



I foot o 

 ostrich. X Hs- 



H, lobate foot of coot. X %. /, clinging foot of flicker. X /"a- 



X ^i- G, syndactyl foot of kingfisher. X % 



J, running foot of 



ture, the bird perspires into the air sacs. Thus during flight, when a 

 great deal of heat is produced, the temperature is regulated by a very 

 perfect internal cooling device. There are no skin glands in the bird 

 except the oil gland at the base of the tail, the secretion of which is used 

 in oiling and dressing the plumage. 



431. Modifications of Birds. — Birds show less variation than any 

 other class of vertebrates, but within narrow limits they exhibit a large 



