Effects of Changing Forest Structure on Long-Term Trends in Bird Populations in SW Finland

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Abstract
Nine forest habitats were studied on Aland, southwest Finland, in 1926-1927 and 1975. Part of the differences between the censuses may be accounted for by differences in census methods, but many changes in bird populations could be predicted on the basis of changing forest structure: the increased coverage of the bush layer after the abandonment of forest grazing and the increase of spruce, scattered birch and the herb layer in certain habitats seemed important. Increased forest fragmentation may also have played a significant role. Other causes include the mild winters preceding 1975 and changes due to new species immigrated to the insular study area (e.g., Carpodacus erythrinus). Non-passerines decreased, but passerines greatly increased their populations. Many species increased strikingly, e.g., Turdus iliacus, Prunella modularis and Anthus trivialis. Species diversity increased in all habitats except one, probably due to increased foliage height diversity (increased bush and herb layers) and to increased horizontal heterogeneity of the habitats (more small habitat patches were studied in 1975 than in 1926-1927).