Forest bat activity declines with increasing wind speed in the proximity of operating wind turbines

The increasing use of onshore wind energy is leading to an increased
deployment of wind turbines in structurally rich habitats such as forests.
Forest-affiliated bats, in turn, are at risk of colliding with the rotor
blades. Due to the legal protection of bats in Europe, it is imperative to
restrict the operation of wind turbines to periods of low bat activity to
avoid collisions. However, bats have also been observed to avoid wind
turbines over several hundred meters distance, indicating a displacement
that cannot solely be explained by modifications to the habitat. This
avoidance suggests a displacement of bats by indirect factors related to
wind turbine operation, e.g., wake turbulences and noise emissions.
Therefore, we investigated whether the activity of forest-affiliated bats
is influenced by operation mode (on/off) under variable wind conditions
along transects from 80 to 450 m distance to wind turbines. We divided
recordings by foraging guild, i.e., either narrow-space (Myotis,
Plecotus), edge-space (Pipistrellus, Barbastella), or open-space foraging
bats (Nyctalus, Eptesicus, Vespertilio), and analyzed the effects of wind
turbine operation and wind speed on the recorded bat guild activity with
mixed effects models. The acoustic activity of narrow-space foraging bats
decreased by 91% with increasing wind speed when wind turbines were
operating, while bat activity remained unaffected by wind speed when
turbines were not operating. This was neither observed for open-space
foraging bats nor for edge-space foraging bats, and neither wind turbine
operation nor wind speed (ranging between 0 – 4 m/s at 10 m height above
ground) were found to affect bat activity when considered alone. Wind
turbine noise emissions are known to increase with rotor speed and
consequently, wind speed, thus presenting a likely explanation for the
interactive negative effect of turbine operation and wind speed
specifically on noise-sensitive narrow-space foraging bats. To understand
potential ecological long-term consequences for bat populations in forest
areas with wind turbines and to design effective conservation measures,
future research should focus on disentangling the effects of different
disturbances related to turbine operation.