Chap. XIII.] DOUBLE ANNUAL MOULT. 77 



of stairs, and the shore of an island in a lake in Nevada 

 was found covered with these curious exuviae." 



Changes of color in the plumage according to the sea- 

 son depend firstly on a double annual moult, secondly on 

 an actual change of color in the feathers themselves, and 

 thirdly on their dull-colored margins being periodically 

 shed, or on these three processes more or less combined. 

 The shedding of the deciduary margins may be compared 

 with the shedding by very young birds of their down ; 

 for the down in most cases arises from the summits of the 

 first true feathers. 73 



With respect to the birds which annually undergo a 

 double moult, there are, firstly, some kinds, for instance 

 snipes, swallow-plovers (Glareolae) and curlews, in which 

 the two sexes resemble each other and do not change 

 color at any season. I do not know whether the winter 

 plumage is thicker and warmer than the summer plumage, 

 which seems, when there is no change of color, the most 

 probable cause of a double moult. Secondly, there are 

 birds, for instance, certain species of Totanus and other 

 grallatores, the sexes of which resemble each other, but 

 have a slightly different summer and winter plumage. 

 The difference, however, in color in these cases is so 

 slight that it can hardly be an advantage to them ; and it 

 may, perhaps, be attributed to the direct action of the 

 different conditions to which the birds are exposed during 

 the two seasons. Thirdly, there are many other birds the 

 sexes of which are alike, but which are widely different in 

 their summer and winter plumage. Fourthly, there are 

 y>irds, the sexes of which differ from each other in color ; 

 but ihe females, though moulting twice, retain the same 

 colors throughout the year, while the males undergo a 



w Mr. D. G. Elliot, in 'Proc. Zool. Soc.' 1869, p. 589. 

 13 ' Nitzsch's Pterylography,' edited by P. L. Sclater. Ray Soc. 

 1867, p. 14. 



