Diet selectivity and shift of wintering common pochards and tufted ducks in a eutrophic coastal lagoon

Diet selectivity and shift of common pochards (Aythya ferina) and tufted ducks (A. fuligula) were investigated in the eutrophic coastal lagoon, Lake Nakaumi, during three winters (1994–1997). These two diving ducks fed mainly on mussels Musculista senhousia, but used food resources differently. Common pochards foraged larger mussels that were depleted earlier than the smaller ones eaten by tufted ducks. After the mussel biomass decreased in late winter, tufted ducks shifted their diet to clams Ruditapes philippinarum and Crustacea, while common pochards shifted only to the clam. Thus, depletion of mussel biomass in the lagoon affected common pochards earlier and stronger than tufted ducks. We suggest that different use of food resources of each species influenced their wintering patterns. Tufted ducks used the wintering area for a long period, while common pochards moved to the other area when animal foods became less abundant.