Three decades of breeding bird population changes in a pristine Danish forest: climate-driven shifts and biodiversity loss compared to national trends

Abstract Climatic changes are causing general ecosystem changes and species-specific range shifts. To increase our understanding of climate change effects on avian trends and range shifts, this study investigates long-term breeding bird population trends in the pristine Høstemark Forest, Denmark, from 1991 to 2024, in the context of climate change and biodiversity loss. Using territory mapping data from unmanaged pristine forest and national point count data, we examined whether climate-group classifications (leading-edge, neutral, trailing-edge) predict population trends and how a high-quality habitat buffers against climate-driven shifts. Results show significant declines in total bird territories since 1991, while forest-specific bird species abundance remained stable in the Høstemark Forest. Trailing-edge species declined both locally and nationally, while leading-edge species increased locally but not significantly at the national level. Climate-neutral species exhibited local declines despite stable national trends. Species turnover rate appeared to occur more rapidly in the Høstemark Forest compared to the national scale, with both losses of trailing-edge species and gains in leading-edge species. However, despite its ecological quality and stability, the Høstemark Forest experienced a decline in overall bird abundance, suggesting that even pristine habitats can be vulnerable to biodiversity loss, potentially driven by climate change, isolation, and edge effects.