During the Dutch Antarctic Expedition 1990/91 on King George Island, operational temperatures in a colony of Antarctic Terns Sterna vittata were occasionally unexpectedly high (+34°C, figure 1). Compared with Arctic Tern S. paradisaea, the resting metabolic rate values in the resident Antarctic Terns were relatively high (figure 2). As resting metabolism and working capacity are coupled, the high resting metabolic rate values in the Antarctic Tern might he seen in connection with the rough conditions during the Antarctic winter. In contrast, Arctic Terns fly 40,000 km annually to spend their winter in the austral summer. Assuming that a selective pressure exists towards energy requirement minimization in the course of evolution. the costly migratory flight of the Arctic Tern must be compensated for energetically. The differences in resting metabolic rate between the two species might be a clue in explaining the co-existence of both species, which have similar breeding habitats, but different wintering strategies.