The contrasting feeding ecology of great egrets and roseate spoonbills in limnetic and estuarine colonies

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 d 13 C and d 15 N stable isotopes, and their diets were assessed using direct (conventional) methods. The diet of spoonbills consisted of fish, insects, crustaceans, mollusks, and seeds, whereas great egrets fed mainly on fish, 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 confirmed 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 interspe- cific competition during breeding.