34 Dwight, The White-winged Gulls. [f^ 



year behind in their development, becoming white at the first post- 

 nuptial moult through deficiency of pigment, and assuming only 

 at the second postnuptial a plumage that more vigorous birds 

 acquire at the first postnuptial. From this it is evident that it is 

 possible in a very few cases to confuse third winter with second 

 winter birds, and this species illustrates well the difficulties that 

 beset the study of plumages and moults. 



It is further evident that only a small percentage of birds of this 

 species fail to acquire adult plumage by their third winter while a 

 good many of them possess the adult mantle and white body feath- 

 ers of the adult during their second winter, off-color wings and 

 tails alone marking them. It is impossible to estimate with any 

 degree of accuracy what proportion of young birds at each suc- 

 cessive moult pass to a more adult stage of plumage and what pro- 

 portion reassume the feathers of adolescence, but it would seem 

 that the time usually assigned for the attainment of adult plumage 

 is exaggerated. Apparently, females are more backward in assum- 

 ing mature feathers than are the males. 



The sequence of plumages and moults here outlined obtains for 

 all the species under consideration. There is reason, however, 

 for believing that in the smaller species a larger proportion of the 

 birds at the successive moults assume feathers characteristic of the 

 adult than is the case in the larger species. 



Larus leucopterus. White-winged or Iceland Gull. 



This species is perhaps even more Arctic in distribution than 

 glaucus, its breeding range extending from Spitzbergen westward 

 to Greenland and the shores of Baffin's Bay. Thus it is associated 

 throughout its range with glaucus, although seldom moving as 

 far south in winter. Some sixty specimens have passed through 

 my hands, and the sequence of moults and plumages is precisely 

 the same as in the larger glaucus of which it is a small edition. 

 There is, however, no overlapping of dimensions, for even the 

 largest male fails to reach the size of the smallest female glaucus. 



Specimens of adults are rare in collections, for I have found 

 only fourteen in all. Young birds in juvenal plumage do not 



