Density Changes and Niche Differences in Island and Mainland Willow Warblers Phylloscopus trochilus at a Lake in Southern Sweden

Abstract

Food supply, competitor density, feeding technique and feeding places of willow warblers were studied on small islands in Lake Moeckeln, southern Sweden and in similar habitats on the mainland nearby. Small insects and spiders were more abundant than larger specimens. Abundance decreased with increasing distance from ground and increased from May-June. The abundance of insects was slightly higher on the mainland than on islands. This was especially clear for small (< 6 mm) insects. Insects over 12 mm long were common in June, but not recorded in May. From 1976-1978 the combined bird population density did not decline on the mainland but decreased by about 24% on the islands. In the willow warbler the decline was 51% on the islands but only 10% on the mainland. The density of willow warblers was highest on island plots in 1976, but the islands may provide suboptimal habitats for this species. A slight niche expansion, more frequent feeding at and near the trunk, was found for the willow warblers on the islands compared with the mainland. Food catching technique remained unchanged.