THE VERTEBRATE ANIMALS 301 



in number. The swan, seeking its food under water, has a neck 

 containing twenty-three long vertebrae ; the English sparrow, in a 

 different environment, has only fourteen short ones. Some bones, 

 notably the breastbone, are greatly developed in flying birds for 

 the attachment of the muscles used in flight. 



Bill. The form of the bill shows adaptation to a wonderful 

 degree, the bills varying greatly according to the habits of the birds. 



Adaptations in the bills of birds. Could we tell anything about the food of a 

 bird from its bill? Do these birds all get their food in the same manner? 

 Do they all eat the same kind of food ? 



A duck has a flat bill for pushing through the mud and straining 

 out the food; a bird of prey has a curved or hooked beak for 

 tearing; the woodpecker has a sharp, straight bill for piercing 

 the bark of trees in search of the insect larvae which are hidden 

 underneath. 



Birds do not have teeth. The edge of the bill may be toothlike, 

 as in some fish-eating ducks; these, however, are not true teeth. 

 Frequently the tongue has sharp toothlike edges which serve 

 the same purpose as the recurved teeth of the frog or snake. 



Adaptations for Active Life. The rate of respiration, of heart- 

 beat, and the body temperature are all higher in the bird than in 

 man. 



