Male bats sing in the rotor area of wind turbines

A research team led by the Museum of Natural History has demonstrated for the first time that various bat species produce courtship calls within the immediate vicinity of wind turbine rotors, whilst flying around the nacelle. Data from over 80,000 sound recordings taken at nacelle height at six German sites, as well as stereo-thermal 3D reconstructions of bat flight paths, confirm both bat calls and increased bat density in the rotor area. Both findings suggest that, under certain circumstances, bats actively approach wind turbines and provide an explanation for why the risk of collision might increase during the mating season.

Male bats, much like songbirds, sing to attract females and deter rivals, often flying around prominent landscape features. In homogeneous, structurally poor landscapes such as farmland, wind turbines represent attractive structures that males likely use as ‘singing perches’. In doing so, they not only put themselves at risk – their song, which can be heard over relatively long distances, can also attract females to the turbines.

The researchers were able to detect bat song at all study sites and throughout the entire study period; this is therefore not a local phenomenon or an isolated case. Remarkably, the songs of the common noctule and the Natterer’s bat were recorded most frequently. These two species are the ones most commonly found colliding with wind turbines in Germany – a clear behavioural link to the known patterns of collision victims.

“A study by colleagues in the north-western German coastal region shows that during the mating season, female rough-skinned bats are found dead beneath wind turbines more frequently than males,” says Martina Nagy, the study’s lead author. “Our findings provide a plausible explanation for the high proportion of females among the collision victims.” 

Based on the duration of the recorded song, the range of the microphones used and existing knowledge of species-specific flight speeds, the researchers were able to deduce that the bats were not simply flying past the turbines whilst singing, but were instead circling around the nacelles or the tower. Consistent with this, the volume of the song fluctuated periodically (louder/quieter), as would be expected when circling a stationary microphone.

The 3D reconstructions created using two thermal imaging cameras also revealed an extremely clear picture. The density of detected bats dropped sharply as the distance from the nacelle increased. This suggests that the bats were actively approaching the turbine structure itself. It had previously been suspected that bats might approach wind turbines because they were searching for roosts or food. The new findings now show that wind turbines are of interest to bats ready to mate. 

The researchers hope that these new findings can be translated into more effective conservation strategies in the medium term. “Wind energy makes an important contribution to climate protection, but this must not come at the expense of species conservation,” says Mirjam Knörnschild, senior author of the study. “If we understand why bats specifically fly towards wind turbines, operators can shut them down more precisely and in a more species-sensitive manner than before, in order to minimise the number of collision victims.”