Density dependent mortality in Oystercatchers Haematopus ostralegus

Numbers of Oystercatchers Haematopus ostralegus overwintering on the Exe estuary, south Devon, increased over the period 1976 to 1991. Mortality of colour-ringed adults also increased over this period. In this study, we tested for density-dependence in adult Oystercatcher annual and winter mortality. We examined mortality in all adults, using estuary-wide bird counts, and in mussel feeders only, using bird densities on the mussel beds. Results for annual mortality were equivocal, but suggested that mortality was density-dependent, both for all adults and for mussel feeders only, No density-dependence was detected in the winter mortality of all adults, but strong density-dependent mortality was found amongst mussel-feeders. By modelling mussel feeder mortality at different times of year, we also predict that strong density-dependent mortality occurs on spring migration.