Density dependent mortality in Oystercatchers Haematopus ostralegus

Numbers of Oystercatchers Haematopus ostralegus overwintering on the Exe es tuary, south Devon, increased over the period 1976 to 1991. Mortality of co lour-ringed adults also increased over this period. In this study, we teste d 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 fo r annual mortality were equivocal, but suggested that mortality was density -dependent, both for all adults and for mussel feeders only, No density-dep endence was detected in the winter mortality of all adults, but strong dens ity-dependent mortality was found amongst mussel-feeders. By modelling muss el feeder mortality at different times of year, we also predict that strong density-dependent mortality occurs on spring migration.