Dietary relationships among Nearctic and Neotropical migratory shorebirds in a key coastal wetland of South America

Abstract

Dietary requirements influence the structure of shorebird assemblages, and information on diet is a key to understanding why and when particular species of shorebird use an area and how competition for food might shape their geographical distributions. We describe the diet and patterns of use of food resources of four migratory shorebirds at Samborombón Bay, Argentina, one of the most important sites for shorebirds in the western hemisphere. The birds consumed items representing at least 15 taxa but only a few taxa comprised the main resources consumed. The four species of shorebird showed dietary flexibility influenced by season and tidal level. Co-occurring species showed complementary differentiation in two dimensions of their respective trophic niches (taxonomic composition, size-class). The greatest differences in taxonomic composition of the diet were between Red Knots, which fed mainly on molluscs, and the other three species, which fed mainly on polychaetes. Polychaete consumers that co-occurred during autumn (Hudsonian Godwit and White-rumped Sandpiper) focussed on different prey sizes. Alternatively, shorebirds that co-occurred during winter (Hudsonian Godwits and Two-banded Plovers) fed mainly on polychaete of similar size but showed differences in the taxonomic composition of their diets. Interspecific competition for food resources might shape the composition of the shorebird assemblage of our study area during the austral autumn and winter.