The ecology of the Manx shearwater Puffinus Puffinus on Rhum, West Scotland
The aim of this project was to investigate the ecology of Manx shearwaters on Rhum, with particular emphasis on the influence of predation and other extrinsic factors upon breeding success and population dynamics. Brown rats were not found to be active predators of shearwaters on Rhum. Few rats were present in the shearwater colony during the birds' breeding seasons, but larger numbers scavenged on the remains of shearwater eggs and chicks during the autumn and early winter. Rats are apparently prevented from becoming established in the shearwater colony by food shortage in late winter and by the colony's location within Rhum. Golden eagles, peregrine falcons and great black-backed guils do prey upon shearwaters on Rhum but have an insignificant effect upon the shearwater population. Hatching success in the colony varies markedly from year to year and is significantly negatively correlated with the incidence of heavy rain during the incubation period. This arises from the variation in susceptibility to flooding among nesting burrows. The tendency for individual burrows to flood was quantified and was found to have a significant influence upon the probability of successful breeding. Weights at fledging showed a significant seasonal decline, apparently due to increased difficulty for adults in provisioning chicks late in the season. Heavy fledglings are apparently more likely to survive than light ones so that there is selective advantage in breeding as early as possible. The ultimate and proximate control of timing of laying is discussed. There was evidence of year to year variation in feeding conditions around the colony. Chicks grew more slowly in 1985 than in 1984 and were fed less frequently. The implications of these findings to the population dynamics and conservation of the colony are discussed.