Prey depletion and foraging strategy in the Oystercatcher Haematopus ostralegus

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Abstract
The responses of a population of Oystercatchers to their own depletion of their prey, the edible cockle Cerastoderma edule, have been examined in Strangford Lough, Northern Ireland., Cockle stocks were severely depleted each winter as a result of predation by Oystercatchers and about half the birds present in October in the main study area had left by March: the reduction was greater when the initial population of Oystercatchers was high than when it was low. Oystercatchers were initially widespread around the Lough but tended to aggregate by January into a few good sites. Within each site annual differences in the location of feeding effort were correlated with year to year variation in the location of second winter cockles. Within the feeding areas in a bay the birds apparently hunted in the short-term on the basis of expectation, ceasing to crop, the densest cockle beds once an average yield for the area has been removed; this yield was apparently estimated from the local density of the anvils on which captured cockles were broken open. This effect led to marginal areas being depleted of cockles more rapidly than high density areas, so that the birds gradually concentrated their hunting onto an increasingly restricted area of the bay. The birds initially took only second winter and larger cockles but by late winter cockle densities had fallen so much that smaller cockles were accepted: areas previously abandoned or unused for hunting were then economically viable because of the reduced expectations of the birds and were incorporated into the feeding areas. These results are seen as consistent with Royama's profitability theory.