Concern over bats being killed by onshore wind turbines has been growing for several years and that worry has just been further reinforced by research in Germany.
Wind turbines attract bats, apparently, or at least one species.
They seem to appear particularly appealing to female noctule bats in early summer, according to a pilot study at German Leibniz Institute for Zoo and Wildlife Research in Berlin.
This fatal attraction was noticed when the flight paths of noctule bats (Nyctalus noctula) were mapped using the latest GPS tracking devices.
The bats managed to take even seasoned experts by surprise.
The driver behind the study is the conflict between the exploitation of wind energy and the conservation of protected bats in Germany, where onshore wind generation capacity is now considerable and still growing.
In short, it seems there is a conflict between the development of renewable energy sources and the conservation of endangered and legally protected bats; a so-called green-green dilemma.
According to expert estimates, about 250,000 of bats sailing through the night sky are currently dying at wind turbines every year as long as turbines are operated without mitigation measures.
The cause of bat death is either through collision with the rotor blades of turbines or a barotrauma caused by abrupt air pressure changes in the tailwind vortices associated with the moving rotor blades. These abrupt air pressure changes shred the inner organs of bats and kill them instantly, a fact brought to public attention in September 2013 by the UK’s Bat Conservation Trust.
It is claimed that 70% of bats killed by German windfarms are migrating species, including noctules, which also rank among the largest bats flying in the night skies of Europe. The fatality rates are particularly devastating for bat species on their way from north-east Europe, where they reproduce, to southern and western Europe via Germany, where they hibernate during the winter months.
How do bats interact with windfarm facilities?
Where do bats prefer to hunt their favourite insect prey?
What distances do bats fly during the hunt for prey?
How high do they fly anyway?
To answer these questions, the research group working with Dr Christian Voigt of the Leibniz fitted adult noctule bats with miniaturised GPS data loggers. As a test area, the researchers selected a patch of forest in Brandenburg, East Germany. Cultivated land and several wind parks surround this forest patch. The result: In the early days of summer, female bats seem to be virtually fixated on the giant windfarms. The majority of female bats even crossed the wind parks.
Voigt suspects the following: “One explanation considers the fact that bats make their homes in trees. In early summer, having just finished raising their pups, the female bats take off looking for new homes and hunting grounds.
“Conceivably, the bats mistake the windfarm constructions for large dead trees, ideal for serving as bat homes. Our American colleagues have suspected this to be the case for North American bat species already.
“By contrast, male bats generally avoided the wind park facilities and continued to commute between their headquarters and hunting grounds without much variation. These male bats had no reason to venture out. They had already established their quarters earlier in the year.”
The researchers were surprised by the long distances the bats flew on their hunts. On average, female bats spent 1.5 hours in the air and covered almost 30km during their hunt. The average hunting time for males was only one hour, covering just 15km.
A few individuals bats flew up to 250 metres high. However, the hunting excursions of 95% of the bats flew to 140m max above ground.
This is risky business for the bats because in most windfarms, the turbine rotors turn at heights between 70m and 130m.
According to Voigt, small changes in the operation of existing windfarms would be sufficient to minimise bat fatalities and defuse the renewable energy – bat conservation conflict.
Bats only rarely fly at temperatures below 10C and wind speeds above 8m per second. This wind speed is close to the minimum where the net energy production of a wind turbine starts.
It seems that running the turbines only at wind speeds above 8m per second would cause less than 1% loss in terms of electricity generated, a minute loss for the operator.
The required technology for adjusting the operation of wind turbines in this manner already exists and is readily available, according to the research.
Voigt indicated that it would therefore be no big deal to support bat conservation in addition to providing green energy.
Bats are listed as protected species in Germany as well as in the entire EU. They are the only mammals capable of true active flight, and they play a very useful role in the environment.