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 first appears 

 highly probable that the second moult has been gained 

 for the special purpose of throwing off these ornaments. 

 We must, how^ever, 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 Gallinaceae, 

 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 wliich they would 

 otherwise have suffered. 



I conclude, therefore, that the habit of moulting 

 twice in the year was in most or all cases first acquired 



■■'^ See Gould's ' Birds of Great Britaiu.' 



