Abstract
Population changes, breeding biology and adult survival rates of Razorbills were documented on the Isle of May between 1981 and 1987. Counts of individual birds peaked in 1982–83 and by the end of the study numbers were apparently starting to decline, possibly because there were fewer non-breeders. The mean annual adult survival rate was 88.8% and breeding success varied from 0.55 to 0.77 young fledged per pair. There was a weak relationship between timing of breeding and March sea surface temperature but other measures of breeding performance were not related to water temperature nor to an index of herring abundance. Population trends in Razorbills and Guillemots were broadly similar. Both species showed consistently high breeding success but over the narrow range of variation observed, annual differences between the species were not significantly correlated.
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