Prey choice and search speed: Why simple optimality fails to explain the prey choice of Oystercatchers Haematopus ostralegus feeding on Nereis diversicolor and Macoma balthica

Oystercatchers breeding on the saltmarsh of Schiermonnikoog rely on two staple foods during the breeding season: the bivalve Macoma balthica and the worm Nereis diversicolor. Both prey are highly profitable, yet individual birds tend to specialize on either of the two prey species for prolonged periods of time, contradicting the simple or 'classic' optimal prey choice model. Although male Oystercatchers often specialize on Macoma, while females often specialize on Nereis, none of the intensively studied individuals was so inefficient at handling either prey that this could have been the reason for excluding one of the prey from the diet. Furthermore, the two prey did not have different distributions in space, nor could short-term fluctuations in prey availability explain the specialization of individuals. It appears that Oystercatchers hunting for Macoma search at a slower speed and make more pecks 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 burrows to feed on the surrounding substrate. The incompatibility of searching for the two prey at the same time explains why the simple optimal prey choice model does not apply in this and probably many other cases. As argued by Gendron & Staddon (1983), the problem of search speed and the problem of prey choice cannot be treated independently.