Habitat preferences of Sombre Tit (Poecile lugubris) in a karst environment

Sombre Tit (Poecile lugubris) is one of the least studied passerine bird species in Europe, and the least known among Paridae species on the continent. The aim of the study is to identify its habitat preferences in a karst environment. The study took place in 2016-2017 within the "Rayanovtsi" Special Protection Area in Western Bulgaria- a limestone terrain with a mosaic of open areas, shrubs and scattered woodland, featured by sinkholes, rocks and caves. Territory and control plots were identified by means of point counts and territory mapping. To quantitatively describe the habitat structure and the fine-scale land-cover, 17 habitat variables were measured at both territory and control plots (the latter located at unoccupied sites), within a radius (180 m) approximating the territory size of the species. We used generalized additive models (GAMs) to identify the main predictors of species occurrence. Six habitat variables drove Sombre Tit occurrence according to the model: tree cover (quadratic effect), shrub cover (positive/quadratic effects), number of trees along the woodland edge and elevation (negative effects), solar radiation and woodland edge length (positive effects); tree and shrub cover were the most important predictors of species occurrence. Sombre Tits occurred at an average density of 0.36 pairs / 10 ha in the karst-dominated study area. Our results highlight the ecotonal habits of the species, provide a first quantitative description of its habitat preferences, and may be used to integrate the species' requirements into management plans of mosaic landscapes in Natura 2000 sites and other protected areas.