I40 ELEMENTS OF OENITHOLOGY. 



one as regards the most important feathers — those of flight. 

 A structure which, however strong it may be, is so delicately 

 formed as is a feather, must be liable to accidents and must sooner 

 or later wear out. But if the feathers needful for flight were 

 not renewed, then such accidents and wearing out would cause 

 birds to be unable to fly, and therefore, for the most part, un- 

 able to fulfil the conditions necessary for life — that is to say, 

 to obtain their i'ood and escape their enemies. On this account 

 in almost all birds the flying-feathers, or quill-feathers of the 

 wing, are shed gradually and in pairs, so that the moulting birds 

 can retain both their equilibrium and their power of flight. 



The numerous family of Ducks forms an exception to this 

 rule, for most of them shed their quill-feathers almost simul- 

 taneously, and so, for a time, are unable to fly. But then, 

 from their mode of life, they have other resources for concealment 

 and escape, and they can obtain their food in or near water. 

 Aquatic herbage, which is commonly luxuriant, also affords 

 them a ready and efiective shelter. 



Most male Ducks not only change their feathers, but also 

 their colours. This necessitates for them another and second 

 moult, in order to put on their gayer plumes for the next breed- 

 ing-season. 



Very rarely, indeed, birds moult in the spring only, as is the 

 case with the Swallows. 



A great number of birds undergo a double moult, but species 

 which are close allies may differ in this respect, as the Grarden 

 Warbler (Sylvia salicaria) differs, as it is said, from the Blackcap 

 (Sylvia atrieapilla') by moulting twice instead of once. 



But there may be yet more changes. Thus the'Ptarmigan 

 (Lagopus mutus) moults after the breeding-season (as usual), 

 both sexes assuming a grey colour. They then moult again, to 

 become white in winter, and then moult a third time in the 

 spring, to assume their breeding costume once more. The quill- 

 feathers of the wing and tail are not, however, so often changed 

 by them. 



The second moult of Birds is, indeed, often but a very partial 

 one, and sometime's a certain change of plumage may be effected 

 by an alteration in the colour of the feathers themselves — 

 such as is sometimes produced by the shedding of the barbulets 

 or barbicels. 



Besides the annual moult of adult Birds, the young may 

 undergo several changes in order to attain the fully developed 



