The diet of herring gulls during the nesting period in Canadian waters of the Great Lakes

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Abstract
We report the content of 132 boli and 2000 pellets regurgitated by adults and 1749 boli regurgitated for or by chicks in 25 herring gull (Larus argentatus) colonies in Lakes Ontario, Erie, Huron and Superior, between mid-April and mid-July, 1977–1983. Fish were the predominant food in all four lakes. Although 11 families of fish and a minimum of 16 species were identified, 80% of the fish were of two exotic species, the alewife (Alosa pseudoharengus) and the rainbow smelt (Osmerus mordax). The dietary importance of these two fish species reflected their relative abundance within a lake. In addition, representatives of eight orders of insects and 11 families of birds were identified. In the one colony where diets of birds of known sex were quantified, male and female gulls fed on different proportions of alewife and smelt, suggesting the sexes have different foraging strategies. Although some food was scavenged, most was obtained alive. Dietary differences existed between colonies and between lakes, both within and between years. We suggest that diet, contaminant burden, and population size of Great Lakes herring gulls will be affected by fisheries policies which alter the predator–prey dynamics of this exotic-dominated ecosystem.