78 SEXUAL SELECTION : BIRDS. [Paut II. 



change, sometimes, as witli certain bustards, a great 

 cliange of color. Fifthly and lastly, there are hirds the 

 sexes of which difter 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 Avhich the iluif [Madietes 2^^tOnax) offers 

 a good instance. 



With respect to the cause or pur2)ose of the differences 

 in color between the summer and winter plumage, this 

 may in some instances, as with the ])tarmigan,'* serve 

 during both seasons as a pi'otection. 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 tliey 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- 

 0])ed 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 {Triiiga canutus) retained 

 their unadorned winter plumage in the Zoological Gar- 



"'• 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 t)ird 

 is known to suflTer greatly from birds of prey, before it has acquired its 

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

 Sweden,' 18C7, p. 125. 



