Mechanisms shaping the functional diversity of birds’ composition in the primeval forest ecosystem of the Białowieża National Park

Functional diversity of bird assemblages has been disproportionally less recognized than taxonomic diversity. Especially, it has been less assessed in temperate primeval forests, serving as a reference point to the vast majority of forests more or less transformed by human activity. Therefore we aimed to determine the functional diversity of bird assemblages at the three levels of forest ecosystem organization: forest types, forest layers, and forest structures, and to determine mechanisms shaping the functional diversity of bird composition under primeval conditions of the Białowieża National Park (NE Poland). We used observations from a regular network of 96 points within six permanent plots in ash-alder and oak-lime-hornbeam forests, and we calculated functional diversity components and community-weighted mean values of birds’ functional traits. Despite higher taxonomic diversity and functional richness in the ash-alder forest, we found higher values of other functional diversity indices the in oak-lime-hornbeam forest. We also found differences in taxonomic and functional diversity among forest layers. We attributed mechanisms shaping the bird assemblages in ash-alder forests with habitat filtering, while niche partitioning or limiting similarity in oak-lime-hornbeam habitat. Bird assemblages on the crown and emergent layers seem to be shaped by niche partitioning mechanisms, whereas bird composition at shrub and understory levels seems to be shaped by habitat filters. Our study revealed that structural diversity at all three levels is crucial for maintaining the functional diversity of bird assemblages in the primeval forest.