PREY SELECTION BY SPARROWHAWKS, ACCIPITER NISUS - RELATIVE PREDATION RISK FOR BREEDING PASSERINE BIRDS IN RELATION TO THEIR SIZE, ECOLOGY AND BEHAVIOR

Predation is a fundamental ecological process, but there are fe iv stu dies of predation risk for adult, breeding birds. In this study, we qu antified Relative Predation Risk (RPR) for 46 species of passerine bir ds preyed on by sparrowhawks during the breeding season in south-weste rn Sweden. The sparrowhawk is a major predator of smaller birds in Eur asian forests and woodland. Near nesting hawk pairs, prey abundance wa s estimated by line transects. From prey remains at hawk nests, we est imated the frequency of prey species in the diet of hawks. For each ha wk pair, RPR values for different prey species were calculated. The me an RPR for each prey species, based on hawk pairs as sample units, ind icated degree of overrepresentation (positive RPR) Or. underrepresenta tion (negative RPR) Of the prey in the diet. Prey species with a high positive RPR nested mainly near human habitations (villages or towns), at forest edges and in farmland. However, there were marked seasonal changes in RPR. Forest birds, such as the goldcrest and wren, had the highest RPR in April. Later, the hawks seemed to hunt mainly in the ha bitats mentioned above, where prey were more abundant or easier to cat ch, resulting in increasing RPR for the house sparrow, yellowhammer, g reenfinch and other species. RPR values were analysed in relation to p rey body size (mass) and relative density, as well as prey foraging he ight and nest height (categorized from the literature). In addition, t he mean perch height and mean exposure of prey species were quantified in the field. Most predation was due to the smaller male hawks, provi ding food for their mates and young. RPR increased with prey body size up to a mass of about 40 g, then declined with increasing body size ( larger prey being more difficult to catch). RPR decreased with increas ing relative density of prey species. Nest height was not correlated w ith RPR, but foraging height seems to be an important factor: RPR decr eased with increasing foraging height. A similar result was found for mean perch height, when larger prey (over 40 g) and singing individual s were excluded from the field data. RPR increased with mean exposure of prey species when larger prey and singing birds were excluded; a co mbination of perch height and exposure improved the correlation with R PR. Singing birds were generally perched high in the vegetation and ma y not suffer high predation risk, contrary to common belief. The resul ts of this study are discussed in relation to temporal and geographica l variation in RPR and antipredator adaptations in passerine birds.