Article

Food Exploitation Patterns in an Assembly of Estuarine Herons

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Waterbirds (Impact Factor: 0.65). 01/2009; DOI: 10.1675/1524-4695(2008)31[179:FEPIAA]2.0.CO;2

ABSTRACT I examined the food exploitation patterns of three day herons, Great Egret (Ardea alba), Snowy Egret (Egretta thula), Tricolored Heron (E. tricolor) and a nocturnal species, the Black-crowned Night-Heron (Nycticorax nycticorax) that nested together in an estuarine colony. Types and sizes of prey were determined by analyzing the food of nestlings. These data were evaluated in light of relative abundance of prey, which was based on fish collected in nearby tidal creeks. The majority of food items (43%) taken by the heron assembly consisted of a two species of nekton, Mummichog (Fundulus heteroclitus) and Grass Shrimp (Palaemonetes pugio). Among the day herons, dietary compositions overlapped to the extent expected if each individual was foraging randomly. The diet of the Black-crowned Night-Heron diverged significantly from those of the Snowy Egret and Tricolored Heron, but not from that of the Great Egret. Considering relative abundance of different sized Mummichogs, the lengths of dietary Mummichogs overlapped as expected between each heron species. The three day herons coincided in their use of feeding microhabitats, most of which were in the intertidal zone. I found no evidence of social foraging: group sizes of feeding herons were the same as those of herons engaged in other activities, and most birds foraged alone. Foraging birds were rarely aggressive. Similarities (overlaps) in resource exploitation, as measured by type and size of prey and by microhabitat use, were as expected under a null model. Only two of 15 pairwise overlaps were less than expected; two were greater than expected. The four heron species formed an assembly of opportunists, whose members took prey according to abundance. The results provide little evidence that the herons partitioned food resources in any significant way. The lack of competitive or cooperative interactions between herons may be related to the superabundance of a few prey species, a condition probably typical of most estuaries.

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