Chap. XIIL Double Annual Moult. 391 



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, 

 &c, for they acquire their beautiful plumes only during the 

 breeding-season. Moreover, such plumes, top-knots, &c, 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. Bartlett 

 that eight or nine specimens of the Knot (Tringa canutus) re- 

 tained their unadorned winter plumage 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. 80 



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 retaining the same colours throughout the year, we may 

 conclude that the habit of annually moulting twice, has not 

 been acquired in order that the male should assume an orna- 

 mental character during the breeding- season ; but that the 

 double moult, having been originally acquired for some distinct 

 purpose, has subsequently been taken advantage of in certain 

 cases for gaining a nuptial plumage. 



It appears at first sight a surprising circumstance that some 

 closely-allied species should regularly undergo a double annual 



plumage of the ptarmigan is of as statements on moulting, see, on 



much importance to it, as a pro- snipes, &c, Macgillivray, ' Hist, 



tection, as the white winter plu- Brit. Birds,' vol. iv. p. 371; on 



mage; for in Scandinavia, during Glareolae, curlews, and bustards, 



the spring, when the snow has Jerdon, ' Birds of India,' vol. iii. pp. 



disappeared, this bird is known to 615, 630, 683; on Totanus, ibid. p. 



coffer greatly from birds of prey, 700 ; on the plumes of herons, ibid, 



before it has acquired its summer p. 738, and Macgillivray, vol. iv. 



dress : see Wilhelm von Wright, in pp. 4.3") and 444, and Mr. Stafford 



Lloyd, ' Game Birds, of Sweden,' Allen, in the ' Ibis/ vol. v. 1863, p. 



186*7, p. 125. 33. 

 83 In regard to the previous 



