Composition, spatial and temporal variation of Common and Arctic Tern chick diets in the Gulf of Maine

From 1990-1997 we observed food deliveries to Common (Sterna hit-undo) and Arctic (S. paradisaea) tem chicks in the Gulf of Maine; deliveries were obs erved at four Common Tern colonies from 1991-1997 and at three Arctic Tern colonies from 1990-1997. Dietary overlap was high. 0.812. Fish comprised ov er 96% of the prey delivered (frequency) to Common Tern chicks. In contrast , Arctic Tern chicks were fed a diet consisting of 65% fish and 35% inverte brates. Although the two terns fed different proportions of fish, both spec ies relied heavily on white hake (Urophycis tenuis), Four-bearded rockling (Enchelyopus cimbrius) and Atlantic herring (Clupea harengus). American san d lance (Ammodytes americanus) was an important prey for Common Terns at on e island. Overall, Common Terns fed their young a greater diversity of prey than did Arctic Terns. We found significant inter-annual and intercolony d ifferences in the diversity and proportions of prey types. "Boom and bust" years were evident for particular prey. We suspect dietary differences in M aine were due to colony location; however, a detailed study of the relation ship between prey availability and abundance and the foraging ecology of te rns in Maine is needed to elucidate these relationships. We believe long-te rm diet data sets gathered from multiple sites are the best way to characte rize waterbird diets.