SCALE AND INTENSITY OF INTERTIDAL HABITAT USE BY KNOTS CALIDRIS-CANUTUS IN THE WESTERN WADDEN SEA IN RELATION TO FOOD, FRIENDS AND FOES

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 availabili ty and presence of avian predators. Numbers in the nonbreeding season showed monthly averages of 10000 to 25000 birds. Over 100000 knots wer e recorded on three occasions. Knots feed in large flocks, individual birds usually experiencing 4000 to 15000 flock-mates. The Siberian-bre eding/west-African wintering canutus subspecies passed through in late July and early August. Otherwise the Greenlandic/Canadian breeding is landica subspecies was present. Over the period 1964-1992 there were n o clear trends in the number of knots, but canutus-knots were particul arly abundant in July-August 1991, whereas in 1992 both subspecies wer e absent. Macoma balthica was the preferred prey of both subspecies. H ydrobia ulvae, Mytilus edulis and Cerastoderma edule were eaten when M acoma was absent close to the surface of the sediment. As Macoma burie d deeper from July onwards, canutus faced better average feeding condi tions than islandica later in the year. The spatial distribution of kn ots feeding on the intertidal flats around Griend was best explained b y the harvestable biomass of the prevalent prey species in a particula r year and season, i.e. Macoma (main prey when their harvestable bioma ss densities were greater than ca 0.8 g AFDM per m2) and Cerastoderma, and by the avoidance of situations where they run the risk of attack by bird-eating bird& Flocks of knots covered most of the intertidal fl ats in the western Dutch Wadden Sea in a couple of tidal cycles. This is about 800 km2, much larger than the equivalent area used by knots o n their wintering grounds in Mauritania (10-15 km2), a difference that is correlated with prey spectrum, prey availability and predictabilit y.