140 ELEMENTS OF ORNITHOLOGY. 
one as regards the most important feathers—those of flight. 
A structure w hich, 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 food 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 effective 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 Garden 
Warbler (Sylvia salicar ia) differs, as it is said, from the Blackeap 
(Sylvia atricapilla) 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 fee 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 sometimes 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 
