The 1995–1999 results of the mapping technique censuses carried out in permanent plots situated in three types of old-growth primeval BNP stands (ash-alder riverine, oak-hornbeam, mixed coniferous) are presented and contrasted with the data gathered in the same plots in the late 1970s. Most community parameters, such as composition of breeding avifauna, species richness, make-up and cumulative proportion of dominants, remained basically unchanged. Only the overall bird density has increased considerably, by 13–38% in different plots. This has been due to parallel increases in numbers of several species, widely differing in their nesting sites, food requirements and migratory habits. As numbers increased simultaneously in all the plots, the density differences across habitats remained the same, from highest densities in riverine stands at the forest edge (up to 124 p/10 ha), through oak-hornbeam stands, to lowest in the coniferous stands (48–50 p/10 ha). Despite this differentiation the breeding avifauna in individual plots was quite similar (density similarity index exceeding 50% ), indicating that their breeding assemblages constituted samples from a single bird community. In most cases the numerical increases could not be attributed to changes in local environmental factors, such as food resources, weather conditions or changes in habitat structure. Only in the coniferous stands, could habitat changes leading to diversification of their structure (gap formation, increasing number of deciduous trees) have been responsible for increasing species richness and abundance there. The apparent lack of a relationship between changes in bird numbers and the local situation suggests that the factors acting on a larger scale (outside the study area) could have been involved. Despite the directional changes in bird abundance observed in the Białowieża Forest, its breeding bird assemblage, when compared with amplitude of changes recorded over the same period in other areas and habitats, stands out as an example of remarkable stability.
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Vol. 37 • No. 1