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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.

This study is focused on the population ecology of water voles (<I>Arvicola terrestris</I>) in linear habitats in north-east Scotland. The three aims of this study were (a) to assess one ecological method for estimating population size, (b) to identify ecological factors acting on individual spacing patterns, and (c) to make and test predictions about the mating system based on the observed spacing pattern. Data from latrine counts were used to validate a method previously proposed to estimate the population size of water voles. The relationship between latrine counts and vole abundance from sites with a broad range of population size was used in the calibration, and equations were derived from six different regressions with appropriate confidence limits. Due to the uncertainties involved, the calibration appeared to be unreliable for estimating the true population size. Two hypotheses, food-defence and pup-defence, previously proposed to explain territoriality of adult female microtines, were tested experimentally by manipulation of food during the reproductive cycles. Female home range length and daily distance moved were monitored by radio-tracking. The changes observed in home range length, movement, and distance between neighbours, suggested that both food-defence and pup-defence influence the spatio-temporal pattern of territoriality in female water voles. Spacing patterns of adult males and females, and the movement of males in relation to oestrus females, were studied by radio-tracking. A male removal experiment was also conducted to establish the method of home range marking by males. The study showed that intersexual overlap was more pronounced than intrasexual overlap, with males showing more intrasexual overlap. Male with heavier weight and larger distances moved were able to monopolise large numbers of receptive females. Results from the male removal experiment suggested that home range boundaries of males were not maintained by daily contact between neighbours. The pattern of space use by both sexes, and the timing of oestrus in females, predicted that mating system is promiscuity.

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