PREY CHOICE AND SEARCH SPEED - WHY SIMPLE OPTIMALITY FAILS TO EXPLAINTHE PREY CHOICE OF OYSTERCATCHERS HAEMATOPUS-OSTRALEGUS FEEDING ON NEREIS-DIVERSICOLOR AND MACOMA-BALTHICA

Oystercatchers breeding on the saltmarsh of Schiermonnikoog rely on tw o staple foods during the breeding season: the bivalve Macoma balthica and the worm Nereis diversicolor. Both prey are highly profitable, ye t individual birds tend to specialize on either of the two prey specie s for prolonged periods of time, contradicting the simple or 'classic' optimal prey choice model. Although male Oystercatchers often special ize on Macoma, while females often specialize on Nereis, none of the i ntensively studied individuals was so inefficient at handling either p rey that this could have been the reason for excluding one of the prey from the diet. Furthermore, the two prey did not have different distr ibutions in space, nor could short-term fluctuations in prey availabil ity explain the specialization of individuals. It appears that Oysterc atchers hunting for Macoma search at a slower speed and make more peck s per distance searched than do birds hunting for Nereis. This accords with the suggestion that, from the point of view of the Oystercatcher , buried Macoma are more cryptic than Nereis which emerge from their b urrows to feed on the surrounding substrate. The incompatibility of se arching for the two prey at the same time explains why the simple opti mal prey choice model does not apply in this and probably many other c ases. As argued by Gendron & Staddon (1983), the problem of search spe ed and the problem of prey choice cannot be treated independently.