Trophic Niche Breadth and Overlap of Three Egret Species in a Neotropical Mangrove Swamp

Abstract

Empirical studies testing the predictions of theoretical models on interspecific competition are scarce. The present study focused on the effects of competition on trophic niche by: (1) analyzing temporal changes in the diet of Snowy Egret (Egrella thula) on a Brazilian mangrove swamp before and after its breeding site was colonized by two congeners, Little Blue Heron (E. caerulea) and Tricolored Heron (E. tricolor) ; and (2) comparing spatial changes in the diet of the three egret species at the study site with six additional localities from the literature. Nestling boluses were analyzed in the 1993 and 1994 breeding seasons, when Snowy Egret was the only species in the area, and in 1995 and 1996, when the two other species were present. At the study site, although Snowy Egret consumed a greater proportion of molly fish (Poecilia spp.) than the two other egrets, all egrets fed mainly on mollies and shrimps, providing grounds for competition and trophic interference. However, contrary to the predictions of trade-off models, the Snowy Egret's trophic niche enlarged when the area was colonized by the other two egrets, suggesting that environmental variability is more relevant than competition in shaping trophic niche. The three egrets fed on similar resources to those found at other sites collated from the literature. Tricolored Heron appeared the most specialized species, showing a consistent, mangrove-related diet; Snowy Egret had a similar, but more variable diet, and Little Blue Heron showed the greatest trophic diversity, with a highly variable diet between sites. Overall, temporal and spatial variability in the trophic niches of egrets better match an independently-evolved pattern of resource use rather than predictions from competitive models of niche coevolution.