LAUSR

Eastern populations of Semipalmated Sandpipers (Calidris pusilla) have declined severely in recent decades, particularly at wintering grounds in northern South America, where about 80% of the population was lost since the 1980s. This study investigates prey choice of Semipalmated Sandpipers at two sites in major wintering grounds on the coast of Maranhao, northeast Brazil. Prey availability estimated from intertidal sediment samples was compared with bird diet reconstructed from prey remains in droppings. Birds (flocks of 80-90 birds) fed almost exclusively on the polychaete Laeonereis culveri (present in all droppings at both sites; ≥ 97% of total setae found in droppings), the most abundant invertebrate in the intertidal sediments overall, but they avoided the smallest sizes despite abundance in sediments and avoided numerous other species. Setae from two other polychaetes were found in droppings (Glycindes multidens, 53% of droppings; Hermondura tricuspis, 47% of droppings), but these represented < 2% in average of all setae in droppings. The disproportional prevalence of a single prey species in the diet of sandpipers in this study is comparable to well-studied stopover areas. High dependence on a single staple prey at different sites in the migration route can make Semipalmated Sandpipers vulnerable to changes in prey abundance, which may partly explain declines observed at the flyway level.