Diversity of the Bialowieza Forest avifauna in space and time

The Bialowieza Forest (BF) is an extensive and relatively little changed forest complex on the Polish-Belarussian border. Data on the structure and diversity of its breeding avifauna have mostly been collected in primeval old-growth stands, preserved within the Bialowieza National Park (BNP). Mapping censuses repeated in space (plots, replicated in riverine, oak-lime-hornbeam and coniferous stands) and time (permanent plots, 28 years) reveal that the breeding bird assemblages are rich in species (29-52/season in a 25 to 33-ha plot), but of moderate overall density (40-120 pairs/10 ha). The latter is due to low, lower than in man-transformed areas, densities of the majority of particular species. Despite marked changes in vegetation structure, the composition of the breeding bird assemblage does not vary much across different primeval forest types, with a single breeding bird community inhabiting all of them, including treefall gaps. High richness of the BF avifauna stems from the Forest's geographical locality, as well as from a high level of the primeval avifauna retention. Over 95% of the 135-140 pristine species still breed there. Especially species-rich groups are birds of prey, owls, woodpeckers and Sylvinae warblers. Bird communities in the BNP and primeval tropical rain forests share many features, suggesting that natural differences between temperate and tropical forests were less pronounced in the past. The combination of high species richness with low densities of individual species is probably a feature of all pristine forests, independent of climatic zone. The BF avifauna must once have been typical of the ancient European forests and it has become so exceptional chiefly because it has preserved most of its pristine features. The BF thus constitutes an indispensable reference site for future studies of woodland bird biology. To ensure its survival should become a priority. Commercial logging, taking place over 80% of the Polish BF part, severely changes the Forest's structure and strongly affects the birds, especially species which depend on dead wood (Dendrocopos leucotos, Picoides tridactylus) and on old-growth stands. The small area protected as the BNP becomes an increasingly isolated "island", in which preservation of the primeval forest features seems to be impossible. In order to retain them, it is necessary to protect the whole BF area.