Sooty Terns Onychoprion fuscatus on Ascension Island in the south Atlantic are a reproductively isolated population

Population size is determined by the balance of births and deaths, and of immigration and emigration. In order to investigate population dynamics of the Sooty Tern Onychoprion fuscatus population on Ascension Island in the south Atlantic, we focussed on the key component of immigration by measuring the proportions of philopatric and immigrant recruits to the breeding population. The Sooty Tern colony had expanded by 7% following the eradication of Feral Cats Felis silvestris in 2003. Between 1975 and 2002 potential recruits to the breeding colony were ringed as chicks on Ascension Island and in colonies elsewhere in the Atlantic. Searches for these birds as adults breeding on Ascension commenced in June 2002 and continued until December 2008. Of the 600 chicks ringed on Ascension, 36 (or 6%) were recruited into the breeding population and of the 9,482 ringed in other colonies, only one (0.01%) was found breeding on Ascension Island. A further investigation of morphometric measurements revealed that Sooty Terns on Ascension were significantly different from those from neighbouring colonies. The sample of birds from Ascension was drawn from a different population and is possibly of a previously unidentified, sub-species. We conclude that immigration to the breeding population on Ascension is minimal and that they are a reproductively isolated population.