Natural nest-sites of Great Tits (Parus major) in a primeval temperate forest (Białowieża National Park, Poland)

Knowledge of the breeding ecology of the Great Tit Parus major is vast, but almost exclusively concerns birds using nest-boxes. Information on birds nesting in natural conditions is scant. Here, we present the results of the first thorough study on natural nest-sites of the Great Tit. The data, including descriptions of nest-cavity location and dimensions, were collected during 39 breeding seasons in the primeval forest of Białowieża National Park (BNP), Poland. With an excess of available tree-cavities providing a diverse choice of nesting options, Great Tits nested mainly in non-excavated, very deep and spacious cavities with elongated, narrow openings, placed at intermediate heights in living tree trunks. Different sets of tree species were used in different habitats. The pattern of nest-site utilisation by Great Tits in BNP overlapped with that recorded in other areas, but showed niche separation from other non-excavating hole-breeders in BNP. This indicates that Great Tits have core nest-site preferences, which have probably evolved in response to selective forces such as, e.g., risk of predation, flooding, sufficient nest illumination and/or efficient air ventilation.