The influence of rainfall and nest‐site quality on the population dynamics of the Manx shearwater Puffinus puffinus on Rhum

Abstract

The Manx shearwater colony in the mountains of Rhum receives a high average rainfall, around 3000 mm, and nest burrows are subject to flooding. Colony hatching success rates on Rhum and neighbouring Canna are significantly negatively correlated with rainfall amounts during the incubation period. The incidence of heavy rainfall during incubation has a greater influence on hatching success than does total rainfall. Fledging success is unrelated to rainfall. Burrows vary with respect to their susceptibility to flooding during heavy rain. A ‘Flooding Likelihood Index’(FLI) was devised which allowed 100 study burrows to be ranked with respect to their risk of flooding in a given rainfall event. Eggs were significantly more likely to be laid and to hatch in burrows less susceptible to flooding. In 1984 and 1985 the overall breeding success rate in burrows with an FLI lower than the median was more than twice that in those with higher than median FLIs. Male shearwaters showed a significantly greater tendency to move to different burrows following breeding failure in the previous season. This may be an adaptive response to the consistent differences in the quality of individual burrows. The shearwater population on Rhum may be limited by the availability of good quality nesting burrows.