Food and indicators of food availability for common terns on the lower Great Lakes

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Abstract
The food of common terns (Sterna hirundo) nesting on the lower Great Lakes was studied in 1979. In western Lake Ontario 90% of the diet comprised alewife and smelt. In the Niagara River the principal food items were smelt, emerald shiner, common shiner, and bluntnose minnow and in eastern Lake Erie the principal items were smelt, emerald shiner, and trout-perch. Nonfish material was rarely observed. Methods for studying the diets of common terns on the Great Lakes are examined.Three indicators of food availability were examined in 1979 at the Eastern Headland in western Lake Ontario: (1) percentage fish accepted by chicks, (2) foraging time, and (3) chick growth. More food was presented to the chicks than they could eat. Food acceptance levels increased with chick age and brood size and ranged from 43.5 to 91.5%. Foraging trip times were 17.2 min on average and, depending on brood size, tern pairs spent an estimated 45–64% of daylight hours foraging for food for their chicks. Weights of 15-day-old chicks in three-chick broods were higher than those reported for other parts of the breeding range.