Rapid population growth of Great Skuas Catharacta skua at St Kilda: implications for management and conservation

Abstract

The St Kilda archipelago is a World Heritage Site holding internationally important populations of a number of seabird species. Great Skuas first bred at St Kilda in 1963, and numbers there rose steadily until 1990. Since then, a rapid expansion has occurred, with the population size increasing on Hirta, the main island in the archipelago, at a rate of 22.1% per annum between 1994 and 1997. In total, 233 pairs were occupying territories at St Kilda in 1997. A simple model suggests that as many as 50% of the birds recruited to Hirta between 1994 and 1997 were immigrants from other colonies. Comparison of breeding data collected in different areas gave no indication that the higher nest density resulting from the recent influx was having any detrimental effect on annual productivity and it seems likely that Great Skuas will continue to increase, at least in the near future. Given the importance of seabird prey in the diet of Great Skuas at St Kilda, the continued expansion in numbers of this species gives some cause for concern. However, because immigration plays a large role in the population dynamics of Great Skuas at this site, any attempts to limit population growth by small-scale culling of adults or reduction of reproductive output is unlikely to succeed, and might result in a decline in recruitment at other colonies of this rare and vulnerable species.