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In the Greater Black-backed and Lesser Black-backed Gulls, 

 the alteration of colour proceeds in a similar manner. In these 

 the slate-black colour is at first faintly developed on both webs of 

 single feathers of the upper plumage, especially the large scapu- 

 lars, and, becoming darker, very soon supplants every trace of the 

 darker markings of the early plumage. In this case, also, the black 

 colour does not attain to the purity in which it is found in the old 

 birds, but remains until the autumn moult in the condition of 

 a pale, blackish, slate-grey. I have had repeatedly examples of 

 the Lesser Black-backed Gull, in this plumage, offered me as 

 Siberian Gulls (Larus affinis = borealis, Brandt), but the colour 

 of their plumage is invariably much darker than that of the latter 

 species. 



In the above-named Gulls the alteration of colour does not, as 

 in many other cases, commence at the shafts or the edges of the 

 feathers ; but both the light grey and slate-black, though at first 

 very faint, yet appear -to spread immediately over both webs of 

 isolated feathers, and increase in depth and purity in proportion as 

 the brown markings of the winter plumage gradually disappear. 

 This change of colour may be followed with extraordinary clearness 

 in the large scapulars, the posterior flight-feathers, and the greater 

 wing- co verts. 



An alteration of colour of this kind in younger individuals, 

 taking place on parts of the body where the change to the breeding 

 plumage is in old birds effected by moulting, has been observed 

 also in the Lapwing, the Golden Plover, and Asiatic Golden 

 Plover (Charadrius fulvus). It occurs in all probability in many 

 other species of the same genus ; but, under my observation, the 

 largest amount of material was furnished by the above-named 

 species. 



In the old birds of these three species, the light-coloured winter 

 plumage of the breast is replaced in spring by a new growth of 

 feathers of a pure and glossy black, whereas in individuals not 

 quite a year old the black colour of the feathers of the breast is 

 produced by an alteration of colour. This black colour first appears 

 at the lower ends of the feathers, and thence spreads upwards ; it, 

 however, does not, as in old birds, extend over the whole surface of 

 the feathers, but only reaches the middle, the radical halves of the 

 feathers remaining white. At the sides of the head, throat, and fore- 

 neck, however, both old and young birds acquire the black colour of 

 their summer plumage by an alteration of colour. It appears, how- 

 ever, that it is only the stronger individuals among these young birds 

 which assume provisionally a nuptial plumage of this kind ; for only 

 solitary examples, and these invariably very strong birds, are met 



