Activity Pattern and Correlation between Bat and Insect Abundance at Wind Turbines in South Sweden

SIMPLE SUMMARY: Wind power is an important energy system in the global transition towards renewable energy. As new wind farms are erected in increasing numbers, they will have an impact on many organisms, e.g., through habitat changes and collision mortalities. In this study, we measure bat activity, insect abundance, and weather conditions to test the hypothesis that insect abundance attracts bats to wind turbines because of feeding opportunities. We found that the relationship between insect abundance and bat activity was relatively weak, providing some support for the feeding-attraction hypothesis. However, we also found a strong correlation between bat passes and weather conditions. This suggests that stop-regulation based on weather conditions might be a solution to avoid collisions. However, this study highlights some of the problems with defining the limits for stop-regulation, as bat activity may be high also at relatively high wind speeds and low temperatures. ABSTRACT: We present data on species composition and activity of bats during two years at three different wind- turbines, located in south Sweden, both at the base and nacelle height. To test the hypothesis that bats are attracted to wind turbines because of feeding opportunities, insects were sampled at nacelle height at one wind turbine using a suction trap, simultaneously as bat activity were measured. At this wind turbine, we also compared two different technical systems for ultrasound recordings and collect meteorological data. The variation in bat activity was high between nights and between wind turbines. In addition to the expected open-air foraging species (Pipistrellus, Nyctalus, Vespertilio and Eptesicus), some individuals of unexpected species (Myotis, Barbastella, and Plecotus) were found at nacelle height. There was a weak but significant positive relation between bat activity and insect abundance, so the hypothesis could not be rejected, suggesting there might be other factors than insect abundance explaining the frequency of bat visits at the nacelle. We found a strong correlation between bat passes and weather conditions. A reasonable way to mitigate collisions is with stop-regulation. However, this study highlights some of the problems with defining the limits for stop-regulation based on weather conditions.