Predation by Sparrowhawks Accipiter nisus and vulnerability of prey

The relationships between weight, abundance (as a measure of availability), and vulnerability of prey of the Sparrowhawk were studied on the basis of five data sets from the Nordic countries. The purpose of the study was to find out whether the predator showed indications of real prey preference. The mean weight of prey used seemed, in general, to be higher than the mean weight of the available prey. There was, however, no evidence that the average size of prey individuals varied according to the mean size of available prey. The expected negative relationship between the weight and abundance of available prey emerged only in two data sets. There were considerable differences in the relationships between weight, abundance, and vulnerability indices in different data sets. In general, the relationship between weight and vulnerability indices seemed to be not significant. The relationship between the abundance and vulnerability indices was (negatively) significant in two data sets. Weight and abundance factors of prey species together explained 2.2-40.3% of the total variation of vulnerability indices in different data sets. When the effect of the other variable was removed, the contribution of weight was at its highest 21.6%, and that of abundance 39.8% of the total variation. Vulnerability indices of many common species in different data sets varied considerably and the various factors affecting this Variation are discussed. It was concluded that the vulnerability of prey of the Sparrowhawk is due to foraging habits of the predator as well as to characteristics of the habitat and those of the prey species itself The large variability of vulnerability indices in many species suggests that the species-specific characteristics were not important determinants of the predation risk, or that at least some of the indices studied were based on deficient data.