Chap. XV. 
SUMMER PLUMAGE. 
181 
common to both sexes. The female is thus rendered 
more conspicuous during the period of incubation than 
during the winter ; but such birds as herons and egrets 
would be able to defend themselves. As, however, 
plumes would probably be inconvenient and certainly 
of no use during the winter, it is possible that the 
habit of moulting twice in the year may have been 
gradually acquired through natural selection for the 
sake of casting off inconvenient ornaments during the 
winter. But this view cannot be extended to the many 
waders, in which the summer and winter plumages 
differ very little in colour. With defenceless species, 
in which either both sexes or the males alone become 
extremely conspicuous during the breeding-season, — 
or when the males acquire at this season such long 
wing or tail-feathers as to impede their flight, as with 
Cosmetornis and Vidua, — it certainly at flrst appears 
highly probable that the second moult has been gained 
for the special purpose of throwing off these ornaments. 
We must, however, remember that many birds, such as 
Birds of Paradise, the Argus pheasant and peacock, do 
not cast their plumes during the winter; and it can 
hardly be maintained that there is something in the 
constitution of these birds, at least of the Gallinaeese 
rendering a double moult impossible, for the ptarmigan 
moults thrice in the year.^^ Hence it must be con- 
sidered as doubtful whether the many species which 
moult their ornamental plumes or lose their bright 
colours during the winter, have acquired this habit on 
account of the inconvenience or danger which they would 
otherwise have suffered. 
I conclude, therefore, that the habit of moulting 
twice in the year was in most or all cases flrst acquired 
See Gould’s ^ Birds of Great Britain.’ 
