Wesołowski T., Tomiałojć L. 1995. Ornithologische Untersuchungen im Urwald von Białowieża - eine Überischt. (Bird studies in the Bialowieża Forest - an overview). Der Onithologische Beobachter 92: 111-146.

The studies were done in an extensive and relatively little changed forest complex on the Polish-Belarusian border. It harbours a very diverse avifauna. In pristine times its avifauna is thought to have been composed of 135-140 species, over 95 % of which still breed there. Especially rich in species are birds of prey, owls, woodpeckers and Sylviinae warblers. Observations on bird ecology and behaviour were gathered mostly in the primeval stands of the Białowieża National Park. They revealed that: (1) Interspecific competition for resources was of rninor importance in shaping structure and composition of breeding bird assemblage. (2) Densities of numerous species remained well below those recorded in fragmented secondary woodlands, this being a result of saturation of habitats at low densities (Iarge territories) or of relatively frequent occurrence of undersaturation. The latter can be due to low productivity of Białowieża populations (see below) and/or their high mortality outside the breeding grounds or in the non-breeding season. (3) Population fluctuations were often not correlated with the availability of food resources (leaf-eating caterpillars, tree seeds), suggesting that high diversity of alternative food resources in the primeval forest allowed birds to switch from one resource to another without showing symptoms of limitation by food shortage. (4) Breeding losses were among the highest recorded for birds in the temperate zone, this being almost totally due to heavy pressure of nest predators. This, in comparison with other areas, low productivity could result in small numbers of breeding birds (undersaturation). Large territories found in several species rnight serve as means of achieving wide scattering of nests to lower the predatory impact on them.

Key words: Primeval forest, hole nesters, population limitation, undersaturation, territorial behaviour, polygyny, food resources, predation pressure, breeding losses, natural cavities.