78 SEXUAL SELECTION: BIRDS. [Part II. 



change, sometimes, as with certain bustards, a great 

 change of color. Fifthly and lastly, there are birds the 

 sexes of which differ from each other in both their sum- 

 mer 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, 74 serve 

 during both seasons as a protection. When the difference 

 between the two plumages is slight it may perhaps be at- 

 tributed, 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 highly devel- 

 oped 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, frequent- 

 ly checks the development of their secondary sexual char- 

 acters, but has no immediate influence on any other char- 

 acters ; and I am informed by Mr. Bartlett that eight or 

 nine specimens of the Knot (Tringa canutus) retained 

 their unadorned winter plumage in the Zoological Gar- 



74 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 disappeared, this bird 

 is known to suffer greatly from birds of prey, before it has acquired ita 

 summer dress : see Wilhelm von Wright, in Lloyd, ' Game-Birds of 

 Sweden,' 1867, p. 125. 



