Over a period of several years (1978-1981), Arctic terns (Sterna paradisaea) nesting in the inland grassy area on Machias Seal Island had higher hatching and fledging success than those in the peripheral bare-bedrock area. In 1990, the opposite trend was recorded. In 1991, we attempted to determine the source(s) of this variation in breeding success between locations. An egg-exchange experiment was carried out between inland and peripheral sites that effectively eliminated differential egg and (or) chick quality as a source of variation. Following this exchange, success rates no longer differed between the habitats involved, despite the fact that chicks tended to be heavier in the peripheral area. The relative influences of parental quality and habitat quality on breeding success were examined as well. The results suggest that complex interrelationships exist among a variety of external factors (such as weather, food supply, and predation pressure), habitat quality, and the quality of birds choosing to nest in these areas. Breeding success in a given area is determined by the particular combination of these factors, a combination that may vary from year to year, colony to colony, and even in different breeding locations within a single colony in a given year.
