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

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

Why do Oystercatchers Haematopus ostralegus switch from feeding on Baltic tellin Macoma balthica to feeding on the ragworm Nereis diversicolor during the breeding season?

Oystercatchers Haematopus ostralegus breeding on the isle of Schiermonnikoog in the Dutch Wadden Sea: switched from a diet dominated by the bivalve Macoma balthica in late spring to a diet dominated by the annelid worm Nereis diversicolor in early summer. Although all Oystercatchers switched, the timing and the magnitude of the switch differed between individuals. Since searching for Macoma appeared incompatible with searching for Nereis, we expected individuals to search for the prey species yielding the highest intake rate for a given period in the season. Some analyses clearly supported this suggestion, while the results of others were at best ambiguous. Although the density of large Macon!a did not change, the intake rate of Macoma declined during the summer, due to a decline in the capture rate of Macoma. This may have resulted from an increase in the burying depth and a decline in the condition of Macoma, as this forced the Oystercatchers to prey on an increasingly smaller fraction of the Macoma population. Intake rate on Nereis was not related to the density of Nereis, which tripled during the course of the study in 1986, while intake rate seemed to decline in late summer after having reached a peak in early summer. This peak was possibly due to a high surface activity of Nereis. Thus, the diet switch may have been due to an increase in the burying depth and a decline in the condition of. Macoma, or to an increase in the surface activity of Nereis, or both. Problems of interpretation arose primarily from the consistent differences between individuals in prey choice and the lack of independent measures of prey availability