Habitat, Female Age, Organo-Chlorine Compounds and Breeding of European Sparrowhawks

Abstract

This paper examines associations between habitat, female age, organochlorine levels in eggs and breeding performance of A. nisus in Annandale, south Scotland. Breeding was better on low mixed farmland (on productive ground) than on upland sheep range or plantation forest (on less productive ground). This was shown by the earlier laying dates on farmland, larger brood sizes, faster growth of young and by the greater proportion of nests that were successful. Although sparrowhawks were restricted to woods for nesting, some aspects of their breeding performance deteriorated with increase in the extent of woodland around the nesting territory. This trend held both in low farmland and in the area as a whole and was probably due more to the increase in woodland as such than to the associated increase in breeding density. Prey densities were greater in small woods in open country than in continuous forest. Territories in larch and pine were used most often followed in order by those in spruce and broadleaved trees. Breeding performance was generally better in the favored trees, but only for laying dates were these differences significant. The proportion of nests that produced young also rose with increasing amount of tree cover on the territory; this was partly due to lessened predation in the thicker territories. There were fewer adult-yearling, and more yearling-yearling and adult-adult matings than expected if mating were random with respect to age. Yearling females had smaller clutches and broods than older ones, and in farmland their young grew less quickly and their nests were less often successful. The failure of certain nests throughout the population was associated with high levels of DDE and polychlorinated biphenyls in eggs. The amount of shell thinning and egg-breakage was highly correlated with DDE concentrations. Different aspects of breeding performance were most closely related with different factors. The frequency with which territories were used for breeding attempts was more closely linked with tree-species than with any other factor; laying dates and nestling growth rates were associated most strongly with surrounding land use and productivity; sizes of clutches and broods were related most closely to the age of the female; while success or failure of the nest was associated both with habitat factors (especially thickness of cover on territories) and with organochlorine levels in eggs and with the extent of shell-thinning.