vacant nests and other factors influencing nest site selection of birds of prey based on case studies in forest habitats in the forest-steppe and steppe zones of eastern ukraine

Our study was conducted in 2003–2012 in Eastern Ukraine, in the basin of the Seversky Donets river. The total surveyed area was ca. 900 km2 of nesting habitats suitable for raptors. A total of 69 vacant nests were found, i.e. 33.2 % of the total number of nests (208). Nests occupied by recipient species, i.e. the so-called ‘effective nest pool’, were 23–24, i.e. 33.3–34.7 % of the pool of available nests. Up to 25 % of all pairs of raptors depend on the availability of vacant nests of heterospecifics. Ravens (Corvus corax) are the most significant donors of nests: 42.5 % of the pool of available nests is built by this species, and more than 60 % of them are occupied by recipient species. Common Buzzard (Buteo buteo) comes second with 26.09 and 58.3 %, respectively. The most common recipients of nests are Hobbies (Falco subbuteo), Common Kestrel (Falco tinnunculus) and Booted Eagles (Hieraaetus pennatus). The most significant factors that govern occupation of vacant nests by recipient species are: availability of nests in the marginal zone of forest plots, i.e. within 500 m from the forest edge, large distance from human settlements (>1500 m), presence of nests located on trees in the canopy storey, and mature and submature age of forest stands. For seven species considered in our research (n=227), the most important factors were position of nests, in the forest canopy layer, no logging activity within300 m of the nest, no regular human disturbance, and presence of “windows” in the canopy made by fallen trees.