172 STRA Y FEA THERS FROM MANY BIRDS. 



and being replaced. This operation goes on very 

 slowly, so as not to incommode the birds in their flight 

 a proceeding which is directly opposite to that in the 

 Geese, Ducks, Grebes, &c. The Swifts and the Birds 

 of Prey depend upon their wings for very existence, so 

 that this slow and chronic state of moult is the one 

 best adapted to their needs. This singular mode of 

 moulting in the Swifts is another very conclusive proof 

 of the distant relationship of these birds to the 

 Swallows. In the Herons, we have birds which assume 

 many nuptial ornaments in spring, gorgets, plumes, and 

 crests, which drop out or abrade as soon as the pairing 

 season has passed. The Ptarmigan's chronic state of 

 moult is adopted for protective purposes. As is well 

 known, this bird is uniform white in winter plumage, 

 except the tail, which is black narrowly tipped with 

 white, and the lores, which in the male only are black. 

 The wings, breast, belly, and under tail coverts are for 

 the most part permanent in colour. In spring, partly 

 by a moult and partly by a change in the colour of the 

 feathers, the remainder of the plumage becomes black 

 mottled with brown, and in autumn gray mottled with 

 black. The change proceeds so slowly that these two 

 moults overlap each other, and consequently an inter- 

 mediate plumage between the two extremes is acquired. 

 Broadly speaking, there is not much difference exhibited 

 between the colour of the sexes in the birds of this group. 



