Bat flight height monitored from wind masts predicts mortality risk at wind farms

- The most complete dataset of bat flight height distribution is presented.
- A bat collision susceptibility index to wind turbine collisions was calculated.
- The collision susceptibility index is correlated with time spent at blade height.
- Results support the relevance of using wind masts for impact assessment studies.

Bat fatalities by collision or barotrauma at wind farms currently raise high conservation concerns. In many countries, pre-installation acoustic surveys are mandatory in order to assess the impacts of wind farm projects. In this purpose, the use of wind masts to estimate bat activity and hence predict collision risk is highly recommended by conservation committees worldwide. Yet, the degree to which collisions may be predicted from acoustic monitoring at wind masts has been strongly debated.To assess this relationship, microphone arrays were installed on 23 wind masts in order to record and locate bat activity on the vertical axis during 3260 nights. For each species, we also calculated a collision susceptibility index, based on fatality data gathered in the literature and corrected for species abundance. We demonstrate that the collision susceptibility index is correlated with the percentage of bat passes at blade height.The acoustic recordings allowed us to establish a reference for the ratios of flight activity above heights of 20-45 m high for more than 16 European bat species. The correlation we demonstrate here between the percentage of bat passes at height - recorded from acoustic surveys on wind masts - and bat fatalities strongly supports that activity estimates from wind masts are appropriate for wind turbine impact assessments.