1. Worldwide, wind turbines are increasingly being built at forest sites to meet the goals of national climate strategies. Yet, the impact on biodiversity is barely understood. Bats may be heavily affected by wind turbines in forests, because many species depend on forest ecosystems for roosting and hunting and can experience high fatality rates at wind turbines.
2. We performed acoustic surveys in 24 temperate forests in the low mountain ranges of Central Germany to monitor changes in the acoustic activity of bats in relation to wind turbine proximity, rotor size, vegetation structure and season. Call sequences were identified and assigned to one of three functional guilds: open-space, edge-space and narrow-space foragers, the latter being mainly forest specialists.
3. Based on the response behaviour of bats towards wind turbines in open land-scapes, we predicted decreasing bat activity towards wind turbines at forest sites, especially for narrow-space foragers.
4. Vertical vegetation heterogeneity had a strong positive effect on all bats, yet responses to wind turbines in forests varied across foraging guilds. Activity of narrow-space foragers decreased towards turbines over distances of sev-eral hundred metres, especially towards turbines with large rotors and during mid- summer months. The activity of edge-space foragers did not change with distance to turbines or season, whereas the activity of open-space foragers in-creased close to turbines in late summer.
5. Synthesis and applications. Forest specialist bats avoid wind turbines in forests over distances of several hundred metres. This avoidance was most apparent towards turbines with large rotors. Since forests are an important habitat for these bats, we advise to exclude forests with diverse vegetation structure as po-tential wind turbine sites and to consider compensation measures to account for habitat degradation associated with the operation of wind turbines in forests.