Predation by Eurasian sparrowhawks produces density-dependent mortality of wintering redshanks

Abstract

1. This study examined annual differences in redshank Tringa totanus winter mortality caused by predation by Eurasian sparrowhawks Accipiter nisus over an 11-year period at a rocky shore in south-east Scotland. 2. Redshank numbers at the beginning of a winter showed no trend over the study period and winter mortality rates also showed no temporal trend. Mean winter mortality through predation by sparrowhawks was 30.6% for juveniles and 5.6% for adults. 3. The study demonstrated density-dependent winter mortality in both juvenile and adult redshanks due to predation by sparrowhawks. Wind speed was also positively associated with juvenile winter mortality but had a weaker influence on adult mortality. This apparent influence of wind speed on juvenile mortality was not confirmed by correlations between monthly juvenile mortality and weather variables (minimum temperature, rainfall and wind speed). 4. Density-dependent functions differed between adult and juvenile redshanks, consistent with differences in the competitiveness of the two age classes. It is suggested that individual differences in vulnerability to predation arose through differences in individual susceptibility to density-dependent competition during foraging. Companion studies indicated that as bird density increased more birds were forced to spend more time feeding in an area where risk of predation was high.