Unexpected weight gain in resident Antarctic Terns Sterna vittata during the austral winter

Abstract

Antarctic Terns Sterna oittata (Fig. 1) may display two very different migratory behaviours. According to Cooper (1976), the terns breeding on Tristan da Cunha in the South Atlantic fly thousands of kilometers to the coasts of Africa where they moult. Antarctic Terns that inhabit the more southern and colder Antarctic Peninsula region moult on their breeding grounds. An early account of these seemingly resident peninsular terns (Holdgate 1963) leaves little doubt that at least part of the tern population wintered in the vicinity of Arthur Harbor (64°46′S 64°03′W), Anvers Island, west of the Antarctic Peninsula. Watson (1975) stated that adult Antarctic Terns are generally sedentary around many insular breeding stations, moving only to the nearest open water in winter.During the year 1975 through 1978, 19 tern specimens of different sexes and ages were collected at Arthur Harbor in the non‐breeding season near U.S. Palmer Station. An additional eight terns were collected at a sea short distance from Anvers Island during the non‐breeding season in 1985 (Pietz & Strong, in press). I found that the adult birds taken at the height of winter weighed significantly more than the 150‐180g of a normal breeding Antarctic Tern. This unexpected discovery prompted me to examine the weights of an additional 34 specimens that had been collected at or near Anvers Island during several breeding seasons. By comparing the weights of adult terns by sex, age, and collection date (Fig. 2, Table 1) I found that both males and females weighed significantly more in the winter non‐breeding season (April‐September) than in the summer breeding season (October‐March) (t24= 6.57, P < 0.001, and t16= 5.71, P < 0.001, respectively). No significant differences were detected between male and female weights in summer(t23=0.76, P >0.20) or winter (t17=1.16, P0.20). In short, it appears that body‐weights of adult terns rise rather dramatically following breeding, attain a peak in mid‐winter, and then fall at the approach of the next breeding season (Fig. 2). I suggest that this increased body‐weight is an adaptation to the austral winter rather than simply a recovery from weight loss due to energetic costs of breeding and moulting.