CHAr. XIII. DOUBLE ANNUAL MOULT. * 81 



summer-plumage, which seems, wlien there is no change 

 of colour, the most jDrobable cause of a double moult. 

 Secondly, there are birds, for instance certain species of 

 Tetanus and other grallatores, the sexes of which re- 

 semble each other, but have a slightly different summer 

 and winter plumage. The difference, however, in colour 

 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 birds, the 

 sexes of which differ from each other in colour; but 

 the, females, though moulting twice, retain the same 

 colours throughout the year, whilst the males undergo 

 a change, sometimes, as with certain bustards, a great 

 change of colour. Fifthly and lastly, there are birds 

 the sexes of which differ from each other in both 

 their summer and winter plumage, but the male un- 

 dergoes a greater amount of change at each i^current 

 season than the female — of which the Ruff (Machetes 

 pu^nax) offers a good instance. 



With respect to the cause or purpose of the differences 

 in colour between the summer and winter plumage, this 

 may in some instances, as wdth the ptarmigan,^* serve 

 during both seasons as a protection. When the dif- 

 ference between the two plumages is slight it may 

 perhaps be attributed, as already -remarked, to the 



''i The brown mottled summer plumage of the ptarmigan is of as 

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

 for in Scandinavia, during the 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. 



VOL. IL G 



