J'LUMAGE OF BIRDS. 241 



and there are but two series produced in one part of the jaw, and 

 only one in the other, in any warm-blooded animal. Feathers, on 

 the other hand, are devehjped in the course of some days ; tliey 

 attain a length of from one to two feet or more in many Birds, 

 and they are almost all renewed every year, — in some species 

 even twice a year. It may be conceived, then, how much vital 

 energy the organisation of lairds must exercise, and how many 

 dangers must accompany so critical a peric^d as that of the 

 moult. 



The plumage is commonly changed several times before it 

 attains that state which is regarded as characteristic of the adult 

 bird. The time rcfj^uired for this varies from one to five years, 

 and several birds rear a progeny before they acrjuire the plumage 

 of maturity. 



When the male bird assumes a vestment differing in cohmr 

 from the female, the young birds of both sexes resemble the 

 latter in their first y)lumage (Blackbird); but when the adult 

 male and female are of the same colour, llic young have then a 

 ])luniage peculiar to themselves (Swan;. AMicn adult birds as- 

 sume a plumage during the breeding season decidedly different in 

 colour from that which they bear in winter, the young birds have 

 a j>lumage intermediate in the general tone of its colour compared 

 with the two periodical states of the parent birds, and bearing also 

 indications of the colours to be afterwards attained at either 

 ])eriod (liuffj. When both males and females are alike in colour, 

 l)ut species of the genus differ widely in colour, as e. g. the IMack 

 and White Swans, the young of such species are alike and of an 

 intermediate hue. 



Changes in the appearance of the plumage of birds may be 

 j)roduced : — 



By the feather itself becoming altered in colour ; 



Jiy the bird's obtaining a certain nunil)cr of new feathers 

 without shedding any of the old ones; 



By the wearing off of the lengthened lighter-coloured ti])S of 

 the barbs of the feathers on the body, by which the brighter tints 

 of the plumage underneath are exposed ; 



By an entire or partial moulting, at which old feathers are 

 throA\Ti off and new ones produced in their places. 



The first three of these changes are observed in adult birds at 

 the approach of the breeding season ; the fourth change is partial 

 in spring and entire in autumn. 



VOL. TI. R 



