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We documented the composition and structure of the breeding bird assemblage in the primeval temperate forest of the Białowieża National Park (BNP), during 2010–2014, and used 40 years of data to assess patterns of its diversity. We applied an improved version of the mapping technique (a combined mapping method) for forest birds in seven plots located in three old-growth forest types: ash-alder riverine, oak-hornbeam, and mixed coniferous. The composition of the breeding avifauna and species richness remained basically unchanged. Jointly 67 (79% of 40-year total) breeding species were recorded in 2010–2014. Overall 49 (57%) of all species bred in the study plots in more than 35 years, they formed c. 97% of the pairs in the breeding assemblage. The composition of the group of dominants changed slightly; Phylloscopus sibilatrix became much less numerous in comparison to the earlier periods. Anthus trivialis ceased to breed — possibly due to disappearance of its habitat. The numbers of Sylvia atricapilla reached the highest ever level, and those of Columba palumbus and Dendrocopos medius equalled the maxima observed during 40 years. The overall breeding densities did not change significantly but they were substantially lower than in the peak year (2001). Crown insectivores, crown nesters and short-distance migrants remained the most numerous foraging, nesting and migratory groups, respectively. The earlier observed density gradient across habitats — highest in the riverine, lowest in the coniferous stands — was retained. Overall composition of the breeding avifauna did not change during 40 years, and no colonization of forest areas by a new species, nor extinction of a formerly widespread species, except A. trivialis, were observed. Some local changes of species richness occurred, however. Large-scale changes in coniferous habitats due to die-back of Picea abies and appearance of canopy gaps were followed by the increase in species richness there, while disappearance of former sharp forest edges reduced the number of species breeding in the “edge” plots. We suggest that the high constancy of species composition of the breeding bird assemblage in BNP results largely from the interplay of two factors: 1) long-term stability of the forest habitats, causing places suitable in one year to remain so over many seasons, and 2) cross-generational reproducibility of the selection criteria used by the birds in their settlement decisions.