184 ANIMAL BIOLOGY 



birds shed or molt their feathers in the fall, but some 

 birds molt at other times also. Frequently the plumage 

 that replaces an older one is of different color and some 

 birds have a regular alternation of summer and winter 

 plumage. The ptarmigan, for instance, is brown and 

 white in summer, but after the molt in the fall it takes on 

 a coat of pure white feathers. In nearly all birds the 

 first feathers that appear in the young are very different 

 from those that come later. Sometimes as in young 



FIG. 143. Diagram of a bird showing external parts. (Drawn by Miss 

 H. M. Gilkey.) 



chickens the plumage is in the form of a soft coat of down 

 which resembles fine hairs, although it is not composed of 

 hairs but of true feathers. Pin feathers are immature 

 stages in the development of the plumage in which the 

 feather is still surrounded by a sort of sac. 



One function of feathers, like that of the fur of mammals, 

 is to protect the body from cold and wet. The feathers 

 on the wings and tail, however, which are much larger and 

 stronger than those elsewhere, are used as organs of flight. 

 The tail which can usually be spread out and contracted 

 again like a fan is employed, like a rudder, as an organ of 



