Short-term fluctuations in a south-taiga bird assemblage: support for an "individualistic" view

Abstract

Short-term variability patterns among forest breeding bird assemblages were analysed on the basis of annually repeated mapping censuses conducted during 1985-1989 in several study plots in the Valdai Uplands, northwest Russia . For most species, there was no evidence that variability in breeding numbers has been strongly limited by any mechanisms operating in the breeding grounds. Only in the Wood Warbler, in a birchdominated forest, did the number of territories vary less than could be explained by chance. When the data from a variety of forest habitats were pooled and analysed together, a prevalence of parallel density fluctuations among species was found. However, there were considerable interhabitat differences in fluctuation patterns . At the habitat level, the highest variability of total bird numbers, and the significant preponderance of parallel density fluctuations among species were found in the richest assemblage with the highest number of congeneric species (grey alder forest). Even secondary hole-nesters gave no indications of compensatory density fluctuations . There was little evidence of competitive density interactions at the assemblage level. Individualistic responses of species to environment, stochastic processes, predation pressure on the key species, and heterospecific attraction seem to be more important in determining bird assemblage variability and creating interhabitat differences in fluctuation patterns .