Breeding bird dynamics in a primaeval temperate forest: long-term trends in Bialowieza National Park (Poland)

Numerical variation of the breeding bird community, its ecological subunits (guilds) and the 26 most numerous constituent species is analysed over 20 yr period (1975-1994), and related to variation in supply of defoliating caterpillars, tree seed crop, numbers of rodents and winter weather. Maximum rates of increase between two years were less than twofold in most species. Changes in numbers of individual species/guilds were either independent of each other or in parallel. The pattern of numerical variation was not related to taxonomic affinities, type of nest site, food types, or migratory habits. Whole bird community and 13 of 26 species showed long-term increasing trends, only three species declined. Numbers remained basically stable during the first decade, increases were concentrated to the second decade. Except tropical migrants (no long-term change) other migratory groups increased in numbers. None of the measured environmental variables showed parallel trends, which could account for the recorded trends. Long-term trends and year-to-year fluctuations were not correlated. The short-term variation in bird numbers was to some extent correlated with (in decreasing order of importance) abundance of defoliating caterpillars, winter severity, and changes in rodent numbers. No correlation with tree seed crop was found. Overall, a substantial part of the variation in breeding bird numbers could not be explained by that set of factors. This could be due to birds using wider spectrum of food sources than those measured, varying predator pressure and/or strong influences from outside the forest, swamping local relationships.