EXTERNAL PARTS OF BIRDS. —FEATHERS. 89 



which 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 many birds change the colors of their plumage remarkably, 

 without losing or gaining any feathers, by some process which affects the texture of the feath- 

 ers, such as the shedding of the barbicels and booklets, or its pigmentation ; or by such processes 

 combined. The male of our bobolink changes from the buff dress of the female to his rich black 

 suit without losing or gaining any feathers. It is difficult to lay down any rules of moulting 

 for particular groups of birds, since birds very closely related differ greatly in respect to their 

 changes of plumage, and the subject has not yet received the attention its interest and impor- 

 tance should claim for it. The physiological processes involved are analogous to those con- 

 cerned in the shedding of the hair of mammals and the casting of the cuticle of reptiles. 



Plumage-changes with Sex, Age, and Season. Aside from any consideration of the 

 way in which plumage changes, whether by moult or otherwise, the fact remains that most birds 

 of the same species differ more or less from one another according to certain circumstances. The 

 dissimilarity is not only in coloration, though this is the usual and most pronounced difiference, 

 but also in the degree of development of plumes, — their size, form, and texture. Since young 

 birds are those which have not come to sexual vigor ; since breeding recurs at regular periods 

 of the year ; arid since males and females usually differ in plumage, — nearly all the various 

 dresses woyn by different individuals of the same species are con-elated with the conditions of 

 the reproductive system. As the internal generative organs represent of course the essential or 

 primary sexual characters, all those of the plumage just indicated may be properly classed as 

 secondary sexual characters. These are of great importance, not only in practical ornithology, 

 but as the basis of some of the soundest views that have been advanced respecting the evolu- 

 tion of specific characters in this class of animals. The generalizations may be made : that 

 when the sexes are strikingly different in plumage, the young at first resemble the female ; 

 when the adults are alike, the young are different from either; when seasonal changes are great, 

 the young resemble the fall plumage of the parents ; and, further, that when the adults of two 

 related species of the same genus are nearly alike, the young are usually intermediate, their 

 specific characters not being fuUy developed. Specific characters are often to be found only in 

 the male, the females of two related species being scarcely distinguishable, though the males 

 may be told apart at a glance. Extraordinary developments of feathers, as to size, shape, and 

 color, are often confined to one sex, usually the male. The more lichly, extensively, or pecu- 

 liarly the male is adorned, the simpler the female in comparison, as the peacock and peahen. 

 The Wise Man of Late has formulated the several categories of secondary sexual characters, 

 giving the following rules or classes of cases : "1. When the adult male is more beautiful or 

 conspicuous than the adult female, the young of both sexes in their first plumage closely 

 resemble the adult female, as with the common fowl and peacock ; or, as occasionally 

 occurs, they resemble her much more closely than they do the adult male. 2. When the adult 

 female is more conspicuous than the adult male, as sometimes though rarely occurs [chiefly 

 with certain birds of prey and snipe-like birds] , the young of both sexes in their first plumage 

 resemble the adult male. 3. When the adult male resembles the adult female, the young of 

 both sexes have a peculiar first plumage of their own, as with the robin [usual] . 4. When the 

 adult male resembles the adult female, the young of both sexes in their first plumage resemble 

 the adults [unusual]. 5. When the adults of both sexes have a distinct winter and summer 

 plumage, whether or not the male differs from the female, the young resemble the adults of 

 both sexes in their winter dress, or much more rarely in their summer dress, or they resemble 

 the females alone. Or the young may have an intermediate character ; or again they may 

 differ greatly from the adults in both their seasonal plumages. 6. In some few cases the 

 young in their first plumage differ from each other according to sex; the young males re- 



