The nestling diets of four heron species breeding on a small forestry islet in Anzali, Iran, were investigated to determine the dietary segregation among sympatric breeders. Regurgitated samples from Black-crowned Night-Herons (Nycticorax nycticorax), Little Egrets (Egretta garzetta), Cattle Egrets (Bubulcus ibis) and Squacco Herons (Ardeola ralloides) were collected during the breeding season from 23 May to 16 August 2016. A total of 1,606 prey items were identified representing 39 prey categories from nine different classes. Differences were detected among these four ardeid species for the percentage of each prey category. Fish dominated the regurgitates of the Blackcrowned Night-Heron (67%; n = 43 samples), Little Egret (45%; n = 33 samples) and Squacco Heron (53%; n = 10 samples), whereas arachnids and insects combined (53% and 40%, respectively, based on total number of individuals) and insects (61.9% based on weight) were the most common prey items for the Cattle Egret. Although fish items were dominant in Black-crowned Night-Herons, Little Egrets and Squacco Herons, their weight (χ23 = 318, P < 0.001) and length (χ23 = 225.2, P < 0.001) differed significantly. There was some overlap in the diet of all four heron species, especially between Black-crowned Night-Herons and Squacco Herons, with a Pianka Overlap Index equaling 0.92. There was sufficient dietary segregation in terms of number, weight and length of prey items that could contribute to a reduction in competition for limited food resources.
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