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Hydrobiologia (2015) 744:187210 DOI 10.1007/s10750-014-2076-1
PRIMARY RESEARCH PAPER
The contrasting feeding ecology of great egrets and roseate spoonbills in limnetic and estuarine colonies
Vanessa Oliveira Britto Leandro Bugoni
Received: 5 November 2013 / Revised: 28 September 2014 / Accepted: 7 October 2014 / Published online: 17 October 2014 Springer International Publishing Switzerland 2014
Abstract Multispecies colonies of wading birds frequently occur in both freshwater and estuarine environments, in locations with potentially safe places for nesting where the nearby shallow waters provide food for rearing chicks. In 2011/2012, we investigated the feeding ecology of two large-sized waterbirds, the great egret, Ardea alba, and the roseate spoonbill, Platalea ajaja, which breed sympatrically in limnetic and estuarine colonies 65 km apart in southern Brazil. Whole blood from chicks was sampled for d13C and
d15N stable isotopes, and their diets were assessed using direct (conventional) methods. The diet of spoonbills consisted of sh, insects, crustaceans, mollusks, and seeds, whereas great egrets fed mainly on sh, insects, and crustaceans. Bayesian stable isotope mixing models indicated that spoonbills fed exclusively in limnetic habitats, regardless of where they bred, whereas egrets breeding in the estuarine
colony fed on both estuarine and limnetic prey, expanding their isotopic niche considerably. Dietary data conrmed this result, with the diets of egrets in the freshwater colony showing high similarity to the diets of spoonbills in both freshwater and estuarine colonies. The isotopic niche overlap was the lowest between species in the estuarine colony, suggesting that the feeding plasticity of egrets reduces interspecic competition during breeding.
Keywords Diet Pellets Stable isotopes Mixing
models Platalea ajaja Ardea alba
Introduction
Coexistence of sympatric species with similar morphological characteristics and similar ecological roles implies there are some differences in the ways they explore the environment. This was formalized by Hutchinson (1957) in terms of how species occupied n-dimensional hypervolumes of the ecological niches. In waterbirds, differences in foraging patterns, prey preferences, and physiological limitations result in trophic segregation during the breeding period (Croxall et al., 1999; Post, 2008). It has been proposed that colonies are located in places where adults can obtain enough food for chicks and themselves in the surrounding areas, because these birds are central-place foragers during breeding (Brzorad et al., 2004), while minimizing trophic niche overlap.
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