Relationships between colony size, adult non-breeding and environmental conditions for Shags Phalacrocorax aristotelis on the Isle of May, Scotland.

Abstract

The number of Shags nesting on the Isle of May, Scotland, increased at an average rate of 11% from 1918 to 1987. Two major perturbations in colony growth occurred in 1974-76 and 1985-86, when numbers dropped to 46% and 79% respectively of the pre-crash level. In both cases, the crashes were associated with late breeding, low chick production and poor first-year survival rates. Attendance of breeders at the colony and timing of breeding were linked to Herring abundance around the island in February and (timing only) to the number of days of easterly winds in March. Annual chick production and first-year survival rates were each positively related to local Herring abundance and to the mean size of sandeels collected by Puffins rearing chicks on the island, but not to overall sandeel biomass in the southern North Sea. Simulation modelling showed that in both crash periods, a proportion of Shags that had bred in a previous year did not breed; the exact proportion depended on the amount of recruitment, and ranged from 10% to 60% in the years when it occurred. With up to 25% of Shags not breeding at least once in their lifetime after having recruited to the colony, such non-breeding behaviour constitutes a standard feature of Shag biology, probably as an adaptation to maximizing lifetime reproductive success in an annually fluctuating environment.