Selection of foraging sites and invertebrate prey by migrant Semipalmated Sandpipers, Calidris pusilla (Pallas), in Minas Basin, Bay of Fundy

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Abstract
We determined the particle size composition of substrates, the species composition and abundances of mud-dwelling invertebrates, and the distribution, densities, and diets of Semipalmated Sandpipers foraging on different mud flats in Minas Basin, Bay of Fundy, during the birds' southward migration. Substrate types varied between the mud flats used by shorebirds; particle sizes ranged from sandy (coarse to fine sands) to muddy (mainly silt and clay). Of 34 species of invertebrates identified, the polychaetes Spiophanes bombyx (Clarapede) and Heteromastus filiformis (Clarapede) and the amphipod Corophium volutator (Pallas) were the most numerous and widely distributed. Spiophanes and the "bamboo" worm Clymenella torquata (Leidy) were more abundant in coarse sandy substrates whereas numbers of Heteromastus were negatively correlated with grain size. The densities of Corophium were positively correlated with the amount of very fine sand in the substrate. The sandpipers foraged in larger numbers on mud flats where densities of Corophium were greatest and it was the main prey taken at those sites. The birds' diet was more diverse in areas of low Corophium density. It appears that the birds use visual cues for foraging and that prey behavior is particularly important in their choice of prey items.