Effects of island isolation and feral mink removal on bird communities on small islands in the Baltic Sea

1. Predation has been suggested as a major cause affecting survival, reproductive success and behaviour in vertebrate prey populations. The breeding season is a critical phase because the essential difference between selecting the proper nest site may be a question of breeding success and failure. 2. Islands may serve as refuges for ground-nesting birds against natural mammalian predators. However, many of the bird species breeding on islands are currently threatened by introduced predators because they may lack mechanisms to confront the threats of the new predator. 3. We studied the combined effects of an introduced generalist predator, American mink (Mustela vison Schreb.), and island isolation and size on species richness, abundance and equitability of birds breeding on small islands in the outer archipelago of SW Finland, Baltic Sea. The study comprised two mink removal areas (one during 1993-2001 and the other during 1998-2001) and two comparable control areas (during 1994-2001 and 1998-2001), each covering 72-130 km(2), of which between 1.07 and 1.27 km(2) were land area. The mean island size in all areas was < 2 ha. 4. In the two control areas, both the species richness and abundance were highest on the most isolated islands, while isolation did not have obvious effects on these variables in mink removal areas. Species equitability was not influenced by mink removal or island isolation. 5. The distribution of a maritime species, the razorbill (Alca torda L.), has changed dramatically: in the 1973-74, before mink invaded the area, the species was found commonly on less isolated islands than in 1994, after mink invasion. 6. We suggest that in the presence of mink birds may have changed their breeding site selection and started to breed on the most isolated islands, which are not visited by mink as frequently as less isolated islands. Therefore, our results indicate that increasing island isolation may increase the number of bird species and their numbers, because island isolation slows down mink dispersal.