Interference Among Oystercatchers, Haematopus ostralegus, Feeding on Mussels, Mytilus edulis, on the Exe Estuary

Abstract

Oystercatchers H. ostralegus eating mussels M. edulis on the Exe Estuary were aggressive and often stole mussels from each other. A stable and linear dominance hierarchy existed among 10 birds studied in detail. The intake rates (biomass consumed per 10 min foraging) of 6 of 8 birds on which adequate data were obtained, decreased with increasing densities of oystercatchers. This did not happen in the 2 top-dominant individuals, which thus did not seem to suffer from interference. Interference in the subdominants was due to increased intraspecific kleptoparasitism, to decreased capture rates probably caused by avoidance, and, perhaps, to increased time spent in aggression. Subdominants avoided high densities of conspecifics, but they did not stop feeding when high densities occurred. The data can be accounted for in terms of the increased opportunities for dominants to steal food from subdominants at high bird densities. Competition for small-scale feeding sites may have been important, too. The dispersion of oystercatchers over the mussel bed probably can be described by a modification of the ideal despotic 10 distribution proposed by Fretwell (1972).