Bird Populations in Pristine Danish Forest Decline Despite Stable Habitat
Three decades of monitoring in Høstemark Forest reveal climate-driven shifts and biodiversity loss even in undisturbed woodlands
Long-term study in a rare intact forest
A 33-year study of breeding birds in Denmark’s Høstemark Forest has revealed that even pristine, unmanaged woodland is not immune to biodiversity decline. Researchers tracked bird populations from 1991 to 2024, comparing local trends with nationwide data, to test how climate change and habitat quality interact. Despite its stable conditions and old-growth characteristics, the forest experienced a significant reduction in overall bird territories.
The research, published in the *Journal of Ornithology*, highlights that high-quality habitats may buffer certain declines but cannot fully protect against broader climate and land-use pressures. “Even in one of Denmark’s best-preserved forests, bird populations are shrinking,” the authors note, underlining the vulnerability of avian communities in a rapidly changing environment.
Winners and losers in a warming climate
The study divided species into three groups: trailing-edge (adapted to cooler conditions), leading-edge (favoured by warmer conditions), and climate-neutral species. Trailing-edge species, such as Willow Warbler, Wood Warbler, and Pied Flycatcher, showed marked declines both locally and nationally. Leading-edge species, including Common Chiffchaff and Short-toed Treecreeper, increased in Høstemark, often colonising the forest during the study period. Neutral species, such as Great Tit and Wood Pigeon, surprisingly declined in the forest even while remaining stable or increasing nationally.
These findings suggest that climate change is pushing species turnover at a faster rate locally than at the national scale. While some species gain ground, others disappear, creating a shifting bird community that masks underlying biodiversity loss.
Species turnover but declining abundance
Although overall species richness appeared relatively stable, with losses offset by new colonisations, the total number of breeding territories fell significantly from 1991 to 2024. This means that while the number of different species remained similar, there were fewer birds overall. Species such as Starling, Crested Tit, and Siskin vanished from the forest, while newcomers included Long-tailed Tit and Common Redpoll.
The authors warn that stable species counts can hide ecological homogenisation – where newly arrived species fail to replace the ecological roles of those lost. This could reduce ecosystem resilience in the long term.
Implications for conservation
The findings underline the limits of small, isolated protected areas in buffering against climate change. Surrounded by agriculture and fragmented landscapes, even Høstemark’s 574 hectares may be too small to resist wider environmental pressures. The study calls for larger, better-connected natural areas to sustain bird populations in the face of climate change.
“Pristine habitats like Høstemark Forest remain ecological benchmarks,” the researchers conclude. “But our results show they are not immune to biodiversity loss – emphasising the urgency of safeguarding larger and more connected wild areas.”
Oct 2025
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