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 Schiermon nikoog in the Dutch Wadden Sea: switched from a diet dominated by the bivalve Macoma balthica in late spring to a diet dominated by the anne lid worm Nereis diversicolor in early summer. Although all Oystercatch ers switched, the timing and the magnitude of the switch differed betw een 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 sea son. Some analyses clearly supported this suggestion, while the result s of others were at best ambiguous. Although the density of large Maco n!a did not change, the intake rate of Macoma declined during the summ er, due to a decline in the capture rate of Macoma. This may have resu lted from an increase in the burying depth and a decline in the condit ion of Macoma, as this forced the Oystercatchers to prey on an increas ingly smaller fraction of the Macoma population. Intake rate on Nereis was not related to the density of Nereis, which tripled during the co urse 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 pos sibly 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 o f Nereis, or both. Problems of interpretation arose primarily from the consistent differences between individuals in prey choice and the lac k of independent measures of prey availability.