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 hooklets, 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 difference, 
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; and since males and females usually differ in plumage, — nearly all the various 
dresses worn by different individuals of the same species are correlated 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, 
pit 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 fully developed. Specific characters are often to be found only in . 
the male, the females of two related species being starcely 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 richly, 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. 8. 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 alune. Or the young may have an intermediate character; or again they may 
differ greatly from the adults in both their seasonal pluinages. 6. In some few cases the 
young in their first plumage differ from each other according to sex; the young males re- 
