AVES 



261 



mode of life. The character of the skeleton, the respiratory 

 organs, and, in part, the sense organs and brain are connected 

 with the powers of flight."' 



Variation in Plumage. — The changing of the feathers and the 

 colors of birds is very little understood. The nestling plumage 

 may be so meager that we speak of the young as naked, but the 

 precocial forms, as the grouse, snipes, and ducks, have a thick 

 covering of down. This is followed by what is known as the 



Fig. 212. — A, ArckcBopteryx inacura, restored (Fowler). B, Section of the 

 tail (after Owen). (Romanes.) 



first plumage, which appears more quickly upon the naked than 

 upon the down-covered young and which may be unlike that 

 of either parent. In most land birds this is soon followed by 

 the immature plumage to be worn during the winter. This 

 plumage may be like that of the adult parent of the same sex, 

 or it may be that both immature males and females may resemble 

 the adult female, or they may be unlike either parent. In the 



1 Hertwig, p. 604. 



