Tidal changes in the choice of Nereis diversicolor or Macoma balthica as main prey species in the diet of the Oystercatcher Haematopus ostralegus

Baltic Tellins Macoma balthica and Ragworms Nereis diversicolor, the two main prey species of Oystercatchers Haematopus ostralegus in our study area on Schiermonnikoog (Dutch Wadden Sea:) during the breeding season, require different feeding strategies. Though individual Oystercatchers tended to specialize on either prey species, there was a clear tidal trend in prey choice. The bivalve Macoma predominated in the diet early and late in the tidal cycle, while Nel-eis worms were the major prey over low water. Intake rate of Macoma did not change during the course of the tidal cycle, whereas the intake rate of Nereis markedly increased following emersion and decreased near to the end of the low water period. We surmise that changes in surface activity of Nereis determine the tidal pattern in intake rate. Remarkably, individual Oystercatchers that specialized on Nereis did not concentrate their feeding activity towards the middle of the low water period. As individual Oystercatchers that specialized in Macoma spent a greater proportion of the time foraging early and late in the tidal cycle, it may be argued though that Nereis-specialists were relatively less active during periods that Nereis was less available. Competing demands may have forced Nereis-specialists to feed during periods of low availability of Nereis.