Is bat activity a reliable predictor of mortality at onshore wind turbines? Evidence from a standardized multi-site monitoring in Europe

Reducing bat mortality at wind turbines has become an urgent conservation priority. However, operators often fail to identify and avoid areas of high bat activity when planning wind facilities, despite recommendations from environmental authorities, and existing curtailment strategies still require optimization. Although acoustic monitoring has been suggested as a practical way of assessing collision risk, the relationship between bat activity and fatalities remains poorly understood and lacks scientific consensus. In this study, we analyze a unique long-term regulatory dataset from France, combining standardized nacelle-height acoustic monitoring and systematic carcass searches conducted at 51 onshore wind turbines over five years. We investigated whether ‘exposed’ bat activity, which is defined as acoustic activity recorded at nacelle height during turbine operation, could reliably predict bat fatalities while accounting for turbine characteristics. We also examined how this relationship varies among functional guilds and across biologically relevant seasons. We found a positive association between exposed activity and estimated fatalities. The relationship was stronger in late summer/early autumn than in spring/early summer. However, exposed activity contributed little to overall model performance. We suggest that the low explained variance reflects the substantial spatial and temporal heterogeneity of our dataset, as well as the limited resolution of mortality estimates. Improving mortality monitoring is therefore essential to better characterize activity–fatality relationships and enhance predictive capacity. While acoustic activity alone may be insufficient to predict bat fatalities across broad scales, it remains a valuable indicator for wind facility planning and an important tool for optimizing curtailment strategies.