Written by Jeremy Cresswell
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.
But:
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.
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