82 SEXUAL SELECTION: BIEDS. Part IL 



direct action of the conditions of life. But with many 

 birds there can hardly be a doubt that the summer 

 pkiinage is ornamental, even when both sexes are alike. 

 We may conclude that tliis is the case with many 

 herons, egrets, &c., for they acquire their beautiful 

 plumes only during tlie breeding-season. Moreover, 

 such plumes, top-knots, &c., though possessed by both 

 sexes, are occasionally a little more highly developed in 

 the male than in tlie 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, fre- 

 quently checks the development of their secondary 

 sexual characters, but has no immediate influence on 

 any other characters ; and I am informed by Mr. 

 Bartlett that eight or nine specimens of the Knot 

 (Tringa canutus) retained their unadorned winter plu- 

 mage in the Zoological Gardens throughout the year, 

 from which fact we may infer that the summer plumage 

 though common to both sexes partakes of the nature 

 of the exclusively masculine plumage of many other 

 birds.'^^ 



From the foregoing facts, more especially from 

 neither sex of certain birds changing colour during 

 either annual moult, or changing so slightly that the 

 change can hardly be of any service to them, and from 

 the females of other species moulting twice yet retain- 

 ing the same colours throughout the year, we may con- 

 clude that the habit of moulting twice in the year has 



75 In regard to the previous statements on moulting, see, on snipes, 

 &c., Macgillivray, ' Hist. Brit. Birds,' vol. iv. p. 371 ; on Glareola?, 

 curlews, and bustards, Jerdon, ' Birds of India,' vol. iii. p. 615, 630, 

 683; on Totanus, ibid, p. 700; on the plumes of herons, ibid, p. 

 738, and Macgillivray, vol. iv. p, 435 and 444, and Mr. Stafford Allen, 

 in the ' Ibis,' vol. v. 1863. p. 33. 



