Chap. XIII. DOUBLE ANNUAL MOULT. 83 



not been acquired in order that the male should assume 

 during the breeding-season an ornamental character; 

 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 with closely-allied birds, some species should regu- 

 larly 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 black-cock only 

 once : some of the splendidly-coloured honey-suckers 

 (Nectarinise) of India and some sub-genera of obscurely- 

 coloured pipits (Anthus) have a double, whilst others 

 have only a single annual moult. '^^ But the gra- 

 dations in the manner of moulting, which are known to 

 occur with various birds, shew us how species, or whole 

 groups of species, might have originally acquired their 

 double annual moult, or having once gained the habit, 

 have again lost it. With certain bustards and plovers 

 the vernal moult is far from complete, some feathers 

 being renewed, and some changed in colour. There is 

 also reason to believe that with certain bustards and 

 rail-like birds, which properly undergo a double moult, 

 some of the older males retain their nuptial plumage 

 throughout the year. A few highly modified feathers 

 may alone be added during the spring to the plumage, 

 as occurs with the disc-formed tail-feathers of certain 

 drongos {Bhringa) in India, and with the elongated 

 feathers on the back, neck, and crest of certain herons. 

 By such steps as these, the vernal moult might be ren- 



'^ On the moulting of the ptarmigan, see Gould's ' Birds of Great 

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

 365, 3G9. On the moulting of Anthus, see Blyth, in ' Ibis,' 1867, p. 32. 



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