BIRDS. 443 



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, while 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 ( Ma- 

 chetes 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 seasons as a protection. When the difference between 

 the two plumages is slight it may perhaps be attributed, as 

 already remarked, 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 conclude that this is the case with 

 many herons, egrets, etc., for they acquire their beautiful 

 plumes only during the breeding-season. Moreover, such 

 plumes, top-knots, etc., though possessed by both sexes, 

 are occasionally a little more developed in the male than in 

 the female; and they resemble the plumes and ornaments 

 possessed by the males alone of other birds. It is also known 

 that confinement, by affecting the reproductive system of 

 male birds, frequently checks the development of their 

 secondary sexual characters, but has no immediate influence 

 on any other characters; and I am informed by Mr. Bart- 



* 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 the spring, when the snow has disap- 

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

 it has acquired its summer dress; see Wilhelm von Wright, in 

 Uoyd, " Game Birds of Sweden/' 1857, p. 125. 



