This paper reports the long-term numerical trends of the thirty common forest bird species and explores changes in the community composition in the three main types of old-growth stands in the Białowieża National Park (E Poland, hereafter BNP) over 45 years (1975–2019). We present recent (2015–2019) data on abundance of birds for the seven study plots and pool them with the time series collected since 1975. The numbers of individual bird species strongly fluctuated, with most of the species showing alternating phases (the initial periods of population growth followed by the periods of population decline or stability). The numbers of 19 species increased; maximum growths by c. 3–5% per year included Columba palumbus, Dendrocopos major, Sylvia atricapilla and Regulus ignicapilla. Among a few declining species, Ficedula hypoleuca, Phylloscopus sibilatrix and F. parva experienced the strongest declines, respectively by 4.0%, 2.7% and 2.2% per year. Mostly the same species bred in the plots in the 1970s and in recent years, indicating a stable species pool. The total abundance peaked around 2005, declining thereafter in deciduous stands, but increasing further (along with the species richness) in the coniferous stands. The similarity index between the study plots (beta-diversity) changed little over 45 years; ash-alder and lime-hornbeam stands remained most similar, while coniferous forests stood more apart. The changes found in the old-growth stands of BNP (mostly coniferous fragments) could be partly explained by natural modification of the habitat structure and the processes acting in the large geographical scales, within or outside of the breeding grounds. The long-term studies such as the one in the BNP reported here, provide a basis for the rates of natural turnover in the bird communities in pristine habitats, directly unaffected by human impact.