Which Forest Characteristics Shape Bird Abundance in Central European Forests? A Case Study Based on Common Breeding Bird Survey in Czechia

A large part of European forests is managed using modern forestry techniques, and it is interesting to explore the relationships between forest bird abundance and variables describing the forests from the perspective of forest managers. The aim of our study was to identify these relationships through nation-wide bird monitoring and forestry data in Czechia. After controlling for the effect of altitude, the forest characteristics that exhibited a significant effect on the abundance in the highest number of species are the proportion of conifers, growth phase and tree damage. In the next step, we performed a principal component analysis to reveal general patterns in those species-specific responses. The main gradient obtained was determined by altitude and discriminated species associated with lowland forests (e.g. Common Starling Sturnus vulgaris, Spotted Flycatcher Muscicapa striata) from those associated with highland forests (e.g. Red Crossbill Loxia curvirostra, Tree Pipit Anthus trivialis). This suggests an important role of altitudinal climate zones in shaping forest bird abundance. The second gradient showed a cluster of species with higher abundance in broad-leaved forests with higher stem density, more vegetation layers, and more damaged (e.g. Common Redstart Phoenicurus phoenicurus, European Pied Flycatcher Ficedula hypoleuca). At the opposite end were clustered species with higher abundance in more coniferous forests, lower stem density, fewer vegetation layers and less damaged (e.g. Eurasian Siskin Spinus spinus, Crested Tit Lophophanes cristatus). The second gradient can be explained by forestry practices applied in Czechia. Specifically, broad-leaved forests generally have a longer logging cycle compared to coniferous forests, resulting in generally older stands exhibiting more signs of damage and multiple vegetation layers, while coniferous forests predominantly maintain a single-vegetation layer structure. Bird association with growth phase was independent from altitude and other forest characteristics.