£>6£ Dwight, Plumages of Gulls. [April 



THE PLUMAGES OF GULLS IN RELATION TO AGE AS 

 ILLUSTRATED BY THE HERRING GULL (LARUS 

 ARGENTATUS) AND OTHER SPECIES. 



BY JONATHAN DWIGHT, M. D. 



Plates X-XIV 



It is nearly a score of years since I placed on record (Auk, 1901, 

 pp. 49-63) the fact that Gulls and Terns pass through a perfectly 

 definite series of plumages separated by definite moults. This 

 record, modified only a little in details, is as applicable today, as 

 it was then, to the Gulls of the world, and it is only because I have 

 gathered together new characters for determinibg the age of the 

 so-called "immature" specimens that I have again brought up 

 the subject. 



While each species has pecularities of plumage at different ages 

 that are specific, it must be remembered that not all species at the 

 same age have equally developed plumages, nor do all birds of the 

 same species at the same age have equally developed plumages. 

 The large species require a longer time to attain adult plumage 

 than do the smaller ones; and there is always a percentage, prob- 

 bably a small one, in every species of laggards or backward birds 

 that require a longer time in reaching maturity than do the aver- 

 age individuals. These laggards are a source of confusion and 

 have been largely responsible for wrong estimates of age and of 

 the duration of "immature" plumages. They are apparently 

 about one plumage behind their fellows, but in a series of skins 

 taken at random, it is hardly possible to do more than guess the 

 percentage that deviates from the average plumage. At each 

 successive moult, however, birds advance in their plumage towards 

 maturity, but not equally. We do find, however, some very defi- 

 nite characters that are correlated with age, and by combining 

 them, we find that the smaller Gulls attain fully adult plumage 

 at their first postnuptial or annual moult, which is at the begin- 

 ning of their second year, medium sized Gulls, at the beginning of 

 their third, and large Gulls at the beginning of their fourth year. 

 The percentage of laggards is apparently greatest at the first period 

 of moult and progressively diminishes afterwards. 



