Some species of bats may mistake wind turbines for tall trees, and follow seemingly familiar air flow patterns to their doom.
US researchers used thermal and infra-red surveillance cameras to observe bat behaviour around three wind turbines over three months, and report their results in today’s Proceedings of the National Academy of Sciences..
The surveillance footage showed bats most often approached the wind turbines from downwind, and tended to do so more when the wind speed was lower.
The bats – most commonly migratory tree bats such as the hoary bat (Lasiurus cinereus) – were also more likely to hang around the turbines on moonlit nights.
“Taken together, these observations suggest that bats may orient toward turbines by sensing air currents and using vision, and that air turbulence caused by fast moving blades creates conditions that are less attractive to bats passing in close proximity,” write the researchers.
Lead author and research biologist Paul Cryan, from the United States Geological Survey in Colorado, says the bats may be mistaking the familiar wind flow patterns for those they would normally encounter around a tall tree.
“They don’t have anything in their evolutionary history that would prepare them for something that looks and feels like a tree but isn’t a tree,” says Cryan.
When the wind speed was low, and the turbine was rotating very slowly or not at all, the bats were often observed flying in and around the turbine, investigating the tower, the central box of the turbine – called the nacelle – and would even sometimes chase the blades.
Unfortunately however, this means the bats are in the line of fire when the turbines spin up.
“They get close to the turbine, and then as the wind gusts or starts picking up, that’s when the blades start moving fast enough to hit them and knock them out of the air,” says Cryan, pointing out that the maximum speed reached by the blade tips on wind turbines can exceed 200 kilometres per hour.
Bat fatalities at wind turbines are a particular concern in the United States, and tens of thousands of bats are estimated to be killed at wind turbines each year. Fatalities peak during low-wind conditions in late summer and autumn.
“I’m encouraged by our findings in that we’re starting to understand why these bats might be approaching turbines but the thing we still don’t know is, if they think they’re trees, if that’s really what’s going on, what resource is it that might be drawing them in?” Cryan says.
While bat fatalities do occur at Australian wind turbine sites, these are less of a concern than in the US because they happen less often, and the bat species affected aren’t endangered, says Australian avian specialist Dr Cindy Hull, from Woolnorth Wind Farm Holdings in Tasmania.
“Our fauna are quite different, how they move across the continent is quite different, and so the issues that the Americans have is a lot of collisions with birds and bats [are on] these large migrations that occur and we don’t have that,” says Hull.
“There are concerns about a couple of species in Victoria, and certainly there are precautions around making sure the risk assessments are thorough, but because they’re not listed species, their populations aren’t really low.”
However, she says, research is needed to understand how bats use the landscape and what might be higher-risk areas for the placement of wind turbines.
“The reality is if you’re going to try to minimise the impacts from these sorts of things, you’ve really got to understand what the causes are, and this is a step towards understanding that.”
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