The factors affecting reproductive success and breeding density in a rural population of blackbirds, Turdus merula L
The aim of this thesis was to identify the factors determining reproductive success and breeding density in a rural population of blackbirds occupying contiguous woodland and farmland habitats. Once these factors were identified, an attempt was made to assess the quality of the two habitats in terms of reproductive success. Predation was the major factor affecting reproductive success. There were no significant effects of habitat on predation when habitat was defined as farmland, woodland and woodland edge. When defined in terms of nesting density, high density 'hot-spot' areas had significantly greater nesting cover and lower predation rates than territories in farmland or in the rest of the wood. Parents could adjust their provisioning rates according to chick demand. Consequently chicks in larger broods were not significantly different in weight to chicks in smaller broods. The seasonal change in clutch size is therefore well adapted to conditions for raising nestlings, although there was indirect evidence that female condition may limit clutch size early in the season. The nestlings were fed two main diet types, earthworms and caterpillars, the availability of the former being related to rainfall and temperature and the latter occurring in a seasonal peak. Nestlings fed on predominantly earthworm diets were significantly heavier, thus caterpillars are probably a lower quality prey. Starvation was a minor cause of nestling mortality. There was some evidence that farmland birds were more dependent on earthworms than woodland birds, and consequently only farmland broods showed a significant relationship between weight and rainfall. This conferred no disadvantage to farmland broods, although this may have implications for reproductive success in very dry years. Farmland breeders showed some characteristics of a population in a suboptimal habitat. Breeding density was low on farmland compared with woodland. This in part may have been due to lack of suitable nesting cover. An experiment with artificial nests indicated that predation would be proportionately higher on farmland if nesting density was increased. Year-to-year variations in density across the whole study site paralleled the relative harshness of the preceding winter. Food supplementation prior to the breeding season had no effect on subsequent breeding density or clutch size. It is concluded that farmland is potentially a sub-optimal habitat if subject to different conditions of weather or breeding density than those observed during the three years of this study.