The last remnants of pristine forests in Europe, especially in the nemoral belt of deciduous and mixed deciduous coniferous trees, are unique for understanding the history, composition and structure of plant and animal communities. This is especially true for the Białowieża forest, NE Poland. The biogeographical history of emblematic forest clades of birds, e.g. woodpeckers, tits and nuthatches, is analyzed from molecular phylogenies which allow to determine their pace and mode of origination and differentiation. The flyways which connect temperate and tropical realms played a major role in the origination and differentiation of many extant lineages. Then, large differences in the structure of bird communities between North American and European forests are discussed and explained in the light of the geographic configuration of major land masses and past climatic changes and upheavals throughout the Neogene. From long term in depth studies in this forest, the composition and structure of bird communities may be used as references for understanding and measuring the processes and mechanisms of change in communities of the western managed forests.

This paper synthesizes all available quantitative information on total breeding densities of Estonian forest bird assemblages, as estimated in territory mapping studies since the 1950s. There have been two approaches, one based on multiple visits (typically 7–8), and the other of reduced effort, developed for long-term monitoring. Compared to these mapping methods, line (strip) transects give ca. 30% lower estimates; they also vary widely. After quality checking, a total of 90 mapping estimates were extracted. The estimates were then integrated into ‘typical densities’ by habitat types, after critically assessing data quality and bias. ‘Typical density’ is conceptualized as a range of common density values in actual topographic, soil, forest-age and landscape conditions, under minor recent thinning influence. It also considers 15–20% underestimation of the territory mapping, which was demonstrated using a case study on post-breeding nest searching of three thrush (Turdus spp.) species. The Estonian forest bird densities show monotonous increases along the stand age, following three alternative shapes related to site productivity: a major increase within 100 years in the most productive forests, a delayed increase (also post 100 years) in pine forests of medium productivity, and slow and slight increases at low-productivity sites. Such density estimates can serve as a basis to analyse deviations in particular conditions and be integrated into land-use scenario-modelling tools across landscapes. It is important to keep the territory mapping approaches to bird census in active use and development, since several issues in conservation and ecosystem functioning cannot be addressed by relative assessments.

Long-term breeding bird censuses in Białowieża National Park (BNP), north-east Poland, have revealed rather low densities of birds and high community stability which may be features of near-natural forest conditions. This paper reports on a long-term census of breeding birds in 50 ha of oak woodland in central England which contained both managed and undisturbed areas. New information is provided on breeding bird density and its spatial variation within English mature broadleaved woodland to address two questions: Under what circumstances are English woods likely to support higher densities than those occurring in BNP? Is there evidence that population and community stability is lower in English woodland? The overall bird density was lower than in the BNP oak-hornbeam stands, at the scale of both the whole study area and the undisturbed stands. Densities of three of the five dominant species (Cyanistes caeruleus, Parus major, Troglodytes troglodytes) were lower in BNP, Fringilla coelebs was higher, while Erithacus rubecula was similar. Within the undisturbed stands, densities of seven of 12 species, including T. troglodytes, were similar to those in BNP, two were considerably higher (C. caeruleus, P. major) and one much lower (F. coelebs). Selection of the external woodland edge by several species resulted in relatively high overall densities in the edge zone. The broad composition of the bird community remained fairly stable with the exception of declines in tropical migrants and increases in shrub layer nesters. Temporal abundance patterns of species showed much individuality. Trends of several species appeared to be driven by changes in habitat structure. However, the same species remained numerically dominant or characteristic of the wood throughout the study period. This suggests there was a core of resilient species, much as proposed for BNP. Although caution is needed in drawing conclusions from a single site, with the exception of a small number of species, in recent decades there is unlikely to have been a general pattern of higher densities of birds in English woodland than in BNP. However, some English woods with highly complex vegetation structures, or a high proportion of edge habitat, do have exceptionally high densities (> 100 territories/10 ha).

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.

Predation is one of the most important factors influencing breeding success. Many bird species suffer from high losses because of predators. I studied breeding success and replacement broods in the Red-breasted Flycatcher Ficedula parva. The Red-breasted Flycatcher is a small bird breeding in natural holes in dense deciduous and mixed forests of Europe. Data were collected during twelve breeding seasons in a primeval forest, in the Bialowieża National Park, Northeast Poland, the best preserved and protected area of the Białowieża Forest with a rich community of birds and mammals including predators. Almost half of the broods of the Red-breasted Flycatcher were lost and most of them because of predation. In cases where the brood was lost in the period of egg laying or incubation, females tended to lay a replacement clutch. First and replacement nest-sites differed insignificantly in characteristics, but new clutches were always laid in a new site. Out of 68 broods, in which a loss was found, replacement broods were recorded in 47.1% cases. Replacement clutches were much smaller and pairs raised significantly fewer fledglings. The proportions of successful and lost broods were similar in the first and replacement attempts (56.1%, and 46.7%, respectively). Red-breasted Flycatchers choose shallow nest-sites that allow the females to observe their surroundings. This allows the female to escape from the nest when a predator approaches, increasing the possibility of a repeat clutch and the attainment of breeding success in the same season.

Tree cavities are an essential resource for cavity-dwelling mammals, birds, invertebrates and fungi, and so are important for maintaining forest biodiversity. In North American forests, woodpeckers (Picidae) play a keystone role in cavity creation by excavating holes. However, in European forests many hole-nesting songbirds rely on non-excavated cavities that are formed by fungal decay and compartmentalization after tree damage. Several factors are recognised in initiating non-excavated cavities that are used by hole-nesting birds, including loss of a tree branch or stem breakage, but this topic is poorly studied. Here, we propose that bark stripping by large herbivores (e.g. Red Deer Cervus elaphus and European Bison Bison bonasus) could be another important, and previously overlooked, mechanism for initiating tree cavities that are used by hole-nesting birds. We suggest that, after the initial damage from herbivore bark-stripping, fungal decay can create specific elongated, slit-like cavities, which are particularly important as nest sites for some common forest songbirds. We outline this idea using original observations and evidence from the literature, primarily from the primeval forest in Poland's Białowieża National Park. We also use studies from elsewhere in Europe to show a generally low usage of slit cavities by birds where large herbivores are scarce or absent. We suggest that restoring such animals in European forests could help to restore the abundance and diversity of the tree cavity resource for hole-dwelling species. We encourage future research to investigate this proposal of large herbivores being important agents of tree cavity formation that could enhance biodiversity.