262 



BRANCH CHORDATA 



first, second, or even the third spring the plumage becomes like 

 that of the adult. 



The time of molting varies not only among different species, 

 but often among different individuals of the same species, ac- 

 cording to sex, age, and physical conditions. All birds molt after 

 the nesting season. Some birds lose a few of the body feathers 

 in the next spring before the nesting season, while many lose the 

 body feathers, but not those of the wing and tail. Some change 



Fig. 213. Pterylse and apteria of Callus bankiva (Nitzsch): a, 

 side; 6, dorsal side. 



Ventral 



color by wearing off the fringes of the feather tips, and others 

 at this season are adorned with special nuptial plumes, 1 such as 

 the aigrette of the heron, for which these birds have been so 

 slaughtered. 



There are no periodic molts of the skin, as in reptiles, but the 

 horny layer of the integument undergoes a constant renewal, as 

 in mammals. Some penguins, it is said, "exhibit the old rep- 



1 Chapman, " Bird Life," p. 38. 



