Page:Auk Volume 37-1920.djvu/348

 £>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.