DOUBLE ANNUAL, MOULT. 385 



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 of their down by 

 very young birds; for the down In most cases arises from the 

 summits of the first true feathers." 



With respect to the birds which annually undergo a double 

 moult, there are, firstly, some kinds, for instance snipes, swallow- 

 plovers (GlareolsB), 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, but warmth seems the most probable end 

 attained of a double moult, where there is no change of color. 

 Secondly, there are birds, for Instance, certain species of Totanus 

 and other Grallatores, the sexes of which resemble each other, 

 but in which the summer and winter plumage differ slightly in 

 color. The difference, however, in these cases is so small 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 birds, the sexes of which differ from each 

 other in color; but the females, though moulting twice, retain 

 the same colors throughout the year, whilst the males undergo a 

 change of color, sometimes a great one, as with certain bustards. 

 Fifthly and lastly, there are birds the sexes of which differ from 

 each other in both their summer and winter plumage; but the 

 male undergoes a greater amount of change at each recurrent 

 season than the female — of which the ruff (Machetes pugnax) 

 offers a good instance. 



With respect to the cause or purpose of the differences in 

 color between the summer and winter plumage, this may in 

 some instances, as with the ptarmigan,™ serve during both sea- 

 sons as a protection. When the difference between the two 

 plumages is slight, it may perhaps be attributed, as already re- 

 marked, to the direct action of the conditions of life. But with 

 many birds there can hardly be a doubt that the summer plumage 

 is ornamental, even when both sexes are alike. We may con- 

 clude that this is the case with many herons, egrets, &c., for 



's Nitzsch's 'Pterylography,' edited by P. L. Sclater. Ray Soc. 1867, 

 p. U. 



™ The brown mottled summer plumage of the ptarmigan is of as 

 much importance to it, as a protection, as the white winter plumage; 

 for in Scandinavia, during tlie spring, when the snow has disappeared, 

 this bird is known to suffer greatly from birds of prey, before it has 

 acquired its summer dress: see Wilhelm von Wright, in Lloyd, 'Game 

 Birds of Sweden, 1867, p. 125. 

 2S 



