Scale and intensity of intertidal habitat use by knots Calidris canutus

In August-October 1988-1992 we studied the distribution and abundance of knots Calidris canutus around Griend in the western Wadden Sea, and the extent to which these can be explained by benthic prey availability and presence of avian predators. Numbers in the nonbreeding season showed monthly averages of 10000 to 25000 birds. Over 100000 knots were recorded on three occasions. Knots feed in large flocks, individual birds usually experiencing 4000 to 15000 flock-mates. The Sibe-rian-breeding/west-African wintering canutus subspecies passed through in late July and early August. Otherwise the Greenlandic/Canadian breeding islandica subspecies was present. Over the period 1964-1992 there were no clear trends in the number of knots, but canutus-knots were particu-larly abundant in July-August 1991, whereas in 1992 both subspecies were absent. Macoma balthica was the preferred prey of both subspecies. Hydrobia ulvae, Mytilus edulis and Cerastoderma edule were eaten when Macoma was absent close to the surface of the sediment. As Macoma buried deeper from July onwards, canutus faced better average feeding conditions than islandica later in the year. The spatial distribution of knots feeding on the intertidal flats around Griend was best explained by the harvestable biomass of the prevalent prey species in a particular year an