Disclosing of biotic interactions between sympatric species is important from both theoretical and practical perspectives, especially when the species are of conservation concern. In ecologically similar species, environmental heterogeneity may change the intensity of their competitive interactions due to varying species responses along the environmental gradients. In this study, we analysed the overlap in nesting sites between the internationally protected, mature forest-dwelling Lesser Spotted Eagle Clanga pomarina and the Black Stork Ciconia nigra. The importance of landscape heterogeneity for habitat segregation between these species was also assessed. The nesting sites of 123 pairs of Lesser Spotted Eagles and 78 pairs of Black Storks, located across different landscapes of the Central, Central-Eastern and Eastern Lithuanian ecoregions were described. A series of discriminant analyses were performed to explore regional patterns of habitat differentiation between the species. Habitat differentiation between the species was estimated by niche overlap values (range 0-1, with a value 0.6 suggested to be the threshold between coexistence and competition), which were 0.5, 0.63 and 0.55 for the Central, Central-Eastern and Eastern ecoregions, respectively. Different habitat factors and/or their combinations explained the habitat differentiation between the species in each ecoregion. Although our data shows the coexistence of the Lesser Spotted Eagle and the Black Stork, in certain regions these mature forest-dwelling predators may use too similar habitats and compete for prime sites under specific landscape structures and short supply of suitable patches. Therefore, we propose the necessity of the importance of a regional estimation on biotic interactions when developing conservation programmes and allocating conservation actions within the target region.

Keywords: Clanga pomarina, Ciconia nigra, coexistence, competition, habitat, spatial variation.