BREEDING BIRD DYNAMICS IN A PRIMEVAL TEMPERATE FOREST - LONG-TERM TRENDS IN BIALOWIEZA NATIONAL-PARK (POLAND)

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