Population fluctuations in two northern land bird communities: effects of habitat, migration strategy and nest-site

Abstrakti

Year-to-year variation in the density of breeding land birds was studied in an alpine heath and mountain birch forest area in northern Finnish Lapland during eight years (1979-1986) using the line transect method. In the alpine heath Anthus pratensis was the most numerous species each year. In the birch forest the dominant species was one of the following three species: Phylloscopus trochilus, Carduelisflammea or Fringilla montifringilla . Of the six most common alpine species only three showed a trend toward increasing density during the study period, but of the 14 most common forest species, 13 showed a trend toward increase . In the alpine heath thenumbers of A. pratensis and Plectrophenax nivalis varied more than could be explained by chance. In the birch forest the same was true for C. flammea and F. montifringilla . Especially in the alpine heath the populations of the most common species fluctuated in parallel annually . Species of similar migration strategy (e.g . the long-distance migrants) did not, however, show similar fluctuation patterns, though ground-nesting species did fluctuate in parallel, the synchronizing factor possibly being mustelid predation.