132 GENERAL ORNITHOLOGY part ii 



critical are periods of the change. The first ph;niage is usually 

 worn but a short time ; then another more or less complete change 

 commonly occurs. The moult is as a rule annual ; and in many 

 cases more than one moult is required before the bird attains the 

 perfection of maturity in its feathering. It is well known how 

 different many birds are the first year in their coloration from 

 that afterward acquired. Sometimes changes progress for several 

 years ; and some birds appear to have a period of senile decline. 

 All such changes are necessarily connected, if not with actual 

 moult, as is the rule, then at any rate Avith wear and tear and 

 repair of the plumage. The first plumage being gained, under 

 whatever conditions peculiar to the species, it is the general rule 

 that birds are subject to single or annual moult. This commonly 

 occurs in the fall, when the duties of incubation are concluded, and 

 the well-worn plumage most needs renewal. Many, however, moult 

 tAvice a year, the additional moult usually occurring in the spring- 

 time, when a fresh nuptial suit is acquired ; in such cases the moult 

 is said to be double or setni-annual. Such additional moult is 

 generally incomplete ; that is, all the feathers are not shed and re- 

 newed, but more or fewer new ones are gained, with more or less 

 loss of the old ones, if any. The most striking ornaments donned 

 for the breeding season, as the elegant plumes of many herons, are 

 usually worn but a brief time, being doffed in advance of the 

 general fall moult. A few birds, as the ptarmigan {Lagoims), 

 regularly have even a third or triple moult, shedding many of their 

 feathers as usual in the early autumn, then changing entirely to 

 pure white for the winter, then in spring moulting completely to 

 assume their wedding-dress. As a rule, feathers are moulted so 

 gradually, particularly those of the wings and tail, and so simultane- 

 ously upon right and left sides of the body, that birds are at no 

 time deprived of the power of flight. The first flight -feathers 

 acquired by young birds are usually kept till the next season ; but 

 in those that fly very early, before they are half-grown, as so many 

 gallinaceous birds do, their first weak Aving-feathers are included in 

 the general moult Avhich occurs to young and old in the fall. The 

 duck tribe offer the remarkable case that they drop their wing- 

 quills so nearly all at once as to be for some time deprived of the 

 power of flight. It is quite certain that inany birds change the 

 colours of their plumage remarkably, without losing or gaining any 

 feathers, by some process which affects the texture of the feathers, 

 such as the shedding of the barbicels and booklets, or its pigmenta- 

 tion, or by such processes combined. The male bobolink (Dolichonyx 

 oryzivorus) changes from the buff' dress of the female to his rich black 

 suit without losing or gaining any feathers, a process which is called 

 ,aptosochromatisni. It is difficult to lay down any rules of moulting 



