Chap. XIII.] DOUBLE ANNUAL MOULT. 79 



dens 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 plu- 

 mage of many other birds. 76 



From the foregoing facts, more especially from neither 

 sex of certain birds changing color 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 colors 

 throughout the year, we may conclude that the habit of 

 moulting twice in the year has not been acquired in order 

 that the male should assume during the breeding-season 

 an ornamental character; but that the double moult, hav- 

 ing 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, 

 with closely-allied birds, some species should regularly 

 undergo a double annual moult, and others only a single 

 one. The ptarmigan, for instance, moults twice or even 

 thrice in the year, and the blackcock only once: some of 

 the splendidly-colored honey-suckers (NectariniaB) of In- 

 dia and some sub-genera of obscurely-colored pipits (An- 

 thus) have a double, while others have only a single an- 

 nual moult. 76 But the gradations in the manner of moult- 

 ing, which are known to occur with various birds, show 

 us how species, or whole groups of species, might have 



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

 etc., Macgillivray, ' Hist. Brit. Birds,' vol. iv. p. 371 ; on Glareohe, cu- 

 lews, and bustards, Jerdon, 'Birds of India,' vol. iii. pp. 615, 630, 688,* 

 on Tetanus, ibid. p. 700 ; on the plumes of herons, ibid. p. 738, and Mac- 

 gillivray, vol. iv. pp. 435, 444, and Mr. Stafford Allen, in the ' Ibis,' voL 

 v. 1863, p. 33. 



76 On the moulting ol the ptarmigan, see Gould's • Birds of Great 

 Britain.' On the honey-suckers, Jerdon, • Birds of India,' vol. i. pp. 359, 

 B65, 369. On the moulting of Anthus, see Blyth, in ' Ibis,' 1867, p. 32. 



