THE CHANGING TRADE-OFF BETWEEN FOOD FINDING AND FOOD STEALING IN JUVENILE OYSTERCATCHERS

When juvenile oystercatchers, Haematopus ostralegus, first arrived on the wintering grounds in August and September, they regularly stole mu ssels, Mytilus edulis, from other, mainly older, oystercatchers. By Oc tober, however, juveniles stole far fewer mussels and found almost all their mussels independently for themselves on the mussel bed. Althoug h stealing a mussel was always less profitable than taking a mussel fr om the mussel bed, a simple rate-maximizing optimality model showed th at, in August and September, juveniles increased both their net and gr oss rates of energy intake by stealing because they were rather ineffi cient at foraging for themselves. By October, their greater efficiency at finding good quality mussels, combined with the increased resistan ce of potential victims to kleptoparasitic attacks, resulted in higher intake rates if juveniles stopped stealing mussels and took mussels o nly from the mussel bed. (C) 1998 The Association for the Study of Ani mal Behaviour.