Endangered bats are being killed by wind turbine blades because the air currents are similar to those near tall trees, a study shows.
It’s feared the legally protected mammals are dying while hunting insects that are attracted by the heat generated by the spinning blades.
Thousands of bats have been killed by wind turbines causing a population decline that could cost the farming industry billions each year.
The nocturnal creatures are welcomed by farmers across the world as they eat large numbers of insects that usually damage crops.
This reduces the amount that farmers have to spend on pesticides and saves millions of new plants that could be obliterated by the creepy crawlies.
Over 600,000 bats were killed by wind turbines across the U.S. in 2O12 alone and in the UK the number of bats in areas where they are put up have fallen by 54 per cent.
The researchers say tree-roosting bats suffer higher fatality rates at the sites than other species and peak during low wind conditions.
They used thermal surveillance cameras situated on the ground, near-infrared video, acoustic detectors and radar to monitor bat behaviour at a wind farm in Indiana over several months.
During periods of low wind more bats were sighted near turbines than during gales. The frequency with which they approached from a downwind direction increased with increasing wind speeds – but only when the blades turned slower than normal.
When the blades moved freely the bats approached less frequently from a downwind direction as wind speeds increased.
The results published in Proceedings of the National Academy of Sciences suggest bats orient toward turbines by sensing air currents.
Around three quarters of British bat species are known to roost in trees. The remaining species tend to favour human-made structures because of a lack of suitable and available tree habitat.
Trees provide shelter and attract a diverse range of insect species for bats to feed on. Since bats are not able to bore holes or make nests they use whatever gaps are available.
The researchers believe tree-roosting bats are attracted to turbines because air currents are similar to those around tall trees that harbour insects on their downwind sides or provide sheltered roosting sites.
The findings could explain why they are more vulnerable to wind farms than non-tree-roosting species.
Dr Paul Cryan, of Fort Collins Science Centre in Colorado, and colleagies said: ‘Bats are dying in unprecedented numbers at wind turbines but causes of their susceptibility are unknown.
‘Fatalities peak during low-wind conditions in late summer and autumn and primarily involve species that evolved to roost in trees.’
He added: ‘We discovered previously undescribed patterns in the ways bats approach and interact with turbines suggesting behaviours that evolved at tall trees might be the reason why many bats die at wind turbines.’
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