Egg predation in forest bird communities on islands and mainland

Summary To examine if differences in egg predation rates could explain differences in bird community composition, egg predation was studied in two years on small islands in a South Swedish lake and on the nearby mainland using both natural and artificial nests.In plots with similar vegetation, the combined density of ground- and tree-nesting bird species did not differ between the islands and the mainland. Egg predation rates were similar on islands and the mainland for natural Turdus nests in two years, and for artificial Turdus and Phylloscopus nests. Unmarked and unvisited experimental nests suffered similar rate of egg predation as marked and visited nests. Egg predation rates were higher on natural nests when artificial nests were also put out, increasing the total nest density. Initial egg predation rates in artificial nests were also higher than later when nest density had decreased by 75%.The egg predators involved differed for artificial Phylloscopus nests between the islands and the mainland. Small mammals were apparently responsible for 29% of the predation on the mainland, but none on the islands. Artificial Turdus nests near crow nests suffered from a higher egg predation rate than nests further away from crow nests. Daily survival rates of Turdus nests increased from the laying to the incubation and further to the fledging state.Egg predation can not explain differences in bird community composition between islands and mainland in the present case.