Bats lured to deaths at wind farms ‘because they think turbines are trees’

Bats may be lured to their deaths at wind farms because they think turbines are trees in which they can find shelter, food and sex, according to new research.

The creatures fly towards slow-moving turbines, only to be killed when gusts of wind spin the blades, scientists investigating “unprecedented” numbers of bat deaths at wind farms suggested.

Flashing red lights may need to be installed at wind farms to help prevent the animals making the potentially-fatal mistake, they said.

Bats were “attracted to and actively approach” turbines when they were either stationary or moving only very slowly, according to the researchers from the United States Geological Survey.

About 600,000 bats are estimated to have been killed by wind farms in the US in 2012.

“Bats may not have the cognitive ability to differentiate wind turbines or other tree-like structures from real trees either at a distance or at close range,” the researchers said.

“The simplest explanation for bats closely approaching turbines may be that they are seeking places to roost in what they perceive as trees while migrating.”

The scientists suggested that the central pole of the wind turbine resembled a tree trunk, while blades resembled branches.

These misleading visual signals – “such as similar silhouettes against the night sky” - were compounded by similar airflow patterns generated by the stationary turbines.

Bats were less likely to approach turbines when the blades were spinning quickly, potentially because this created turbulence, according to the scientists.

“Our observations that tree bats show a tendency to closely investigate inert turbines and sometimes linger for minutes to perhaps hours… highlight the plausibility of a scenario in which bats are drawn toward turbines in low winds, but sometimes remain long enough to be put at risk when wind picks up and blades reach higher speeds,” they said.

The scientists suggested one remedy would be to alter the appearance of wind farms, for example by installing lights on the turbines, which might “might make some bats less likely to mistake them for trees”.

They cited the example of one wind farm in Texas where “fewer fatalities of eastern red bats were found under turbines with flashing red aviation lights”.

They suggested that wind farm operators should also only allow the turbines to spin when they wind speeds were consistently high, in order “to prevent gusts from intermittently pushing blades to lethal speed during low-wind periods”.

In a paper, published in the journal Proceedings of the National Academy of Sciences on Monday, the researchers said that as well as seeking shelter, bats may also be lured toward turbines with the expectation of finding “social opportunities or food”.

Bats may head toward what they think are trees in search of a mate, especially as some species of bat carrying out mating displays at trees. The highest death rates from bats at wind farms were documented around the start of the mating season.

The bats may also be drawn toward the tree-like machines with the expectation of finding insects. The researchers said it was not clear whether the bats actually found insects when they arrived at the turbines – as some previous theories have suggested – but that the animals “may be acting upon the expectation of resources rather than the actual presence of resources”.

Other theories have suggested that bats may suffer bends-like symptoms from air pressure changes caused by the turbines, resulting in their internal organs exploding.