The first clue that bats were dying due to a case of mistaken identity was that the dead were mostly tree-roosting species. Wind turbines are killing the tiny flying mammals in record numbers, but the cave dwelling varieties were largely unaffected. Dead bats have turned up at wind turbine facilities on multiple continents, with death totals ranging from tens to hundreds of thousands. Bats are fighting a war on two fronts: one against wind farms, the other against white nose syndrome.

The puzzling part is that bat collisions with tall man-made structures aren’t all that common; they’re mostly limited to wind farms. There must be something unique about wind turbines with their massive spinning blades of doom that actively attracts bats to their eventual demise. At least, that’s what a group of researchers led by Paul M. Cryan of the US Geological Survey’s Fort Collins Science Center, suspected.

Like any good ecological detective, Cryan began by putting together all the clues.

One, three quarters of bats killed by turbines were ones that roosted in trees, at least in North America and Europe.

Two, most fatalities at Northern Hemisphere sites occurred during the late summer and autumn, with a smaller peak in the spring.

Three, while multiple tree-dwelling species were affected by turbine-based mortality, what they had in common was certain behavioral traits. That suggested that bat behavior would play a key role in solving the mystery.

Four, fatalities were more likely when wind speeds were lower than 5-6 meters per second. Early research suggested that bats could be saved if the turbine blades were prevented from turning until winds grew faster than that speed. The problem is that there are financial incentives to keep the turbines spinning even in those lower speeds, despite the tremendous costs to bat populations.

Cryan’s goal, then, was to see what the underlying reasons were for bats’ susceptibility to wind turbines. Perhaps he could find a way to reduce fatalities while simultaneously maximizing power production.

Lots of ideas were put forth to explain the massive mammalian die-offs. Some argued that the deaths were random, with bats being struck by the blades while migrating past wind farms in large numbers. Others offered that bats flapped their wings through wind farms in search or food, shelter, or social opportunities, such as mating.

Cryan and his team conducted an experiment at a wind facility in northwestern Indiana in the late summer and autumn of 2012. The facility was outfitted with infrared video cameras, acoustic detectors, and radar. The cameras would help identify specific bat behaviors that were occurring near the turbines. The acoustic detectors could help determine which species were passing through, since each has its own distinctive calls. The radar, meanwhile, would aid researchers in estimating the overall number of bats flying through, along with whatever other airborne creatures came by.

In all, they collected more than 1300 hours of thermal imagery across 163 nights. The radar detected some 3-4 million animals, and their infrared cameras identified 1261 flying critters comprising 993 bats, and smaller numbers of birds and insects. Nearly 90% of bats caught on camera were seen actively approaching the turbines, which verified the assumption that the bat deaths weren’t simply the result of animals passing through. The bats seemed particularly interested in the nacelles (the unit housing the machinery which turns the blades), and somewhat less interested in the blades and the pole itself.

Despite the fact that millions of avian animals were picked up by the radar, the thermal video cameras primarily picked up bats. That reinforced the notion that bats were actively flying towards the turbines, and also that other animals, like birds, were avoiding the area. Bats also seemed to prefer the downwind side of the turbines. That’s a detail that stood out.

Putting all his data together, Cryan reasoned that tree-roosting bats were attracted to wind turbines because they share some characteristics with trees. They’re tall, with long narrow trunks and massive canopies, made either of branches or of blades.

Aside from their appearance, why do wind turbines act so much like trees? Air may flow similarly around both wind turbines and trees, making it that much easier to mistake one for the other. That helps explain why bats are predominantly struck by turbine blades at low air speeds: once the wind picks up, they stop acting like trees. First, bats may be attracted to that sort of “air profile,” while birds are not, because of particular features of the nacelles that offer good roosting spots. Indeed, hoary bats and eastern red bats were seen approaching the nacelles’ exhaust ports, and video analysis of their flight patterns suggested they were trying to land.

In addition, peak bat mortality is also associated with their mating season, and tall trees act as singles bars for bats. Bats flapping around wind turbines may be looking for love in all the wrong places.

Finally, bats may be drawn towards wind turbines by the insects they eat. Many dead bats have been found with their bellies full, supporting that hypothesis. And since those bugs like to hang out on the downwind side of trees, the bats may be looking for them downwind of the turbines.

It isn’t clear if they’re actually finding insects, or roosts, or mating opportunities on the turbines, but that doesn’t mean they’re not looking. Bats may be “acting upon the expectation of resources,” in other words, rather than the resources themselves.

“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 write, “particularly if visual cues, such as similar silhouettes against the night sky, are accompanied, reinforced, or overwhelmed by other perceptual cues, such as similar downwind airflow patterns.” So what’s the solution?

Perhaps anti-bat measures, such as speakers playing startling sounds, could be installed on turbines, aimed at the space around the nacelle and downwind of the blades. Since bats seem attracted to blades spinning at low speeds, but not at high speeds, safeguards could be installed to limit the blades’ ability to rapidly speed up when the wind picks up. Instead, the turbine could speed up more slowly, giving bats a chance to fly away. Finally, turbines could be outfitted with additional features, such as flashing lights, to make their non-tree status more salient. Support for that idea comes from another study of eastern red bats in Texas. There, turbines with flashing red aviation lights were less likely to strike bats than those without.

Just because a massive wind turbine doesn’t look anything like a tree to us, doesn’t mean that it’s an obvious distinction to a tiny airborne mammal that relies on echolocation and air flow to get around. To avoid killing bats means that we have to climb inside of their heads and try to imagine what it’s like to be one. – Jason G. Goldman | 03 October 2014

Source: Cryan, Paul M. et al. (2014). Behavior of bats at wind turbines. PNAS. doi: 10.1073/pnas.1406672111.

Header image: An eastern red bat, one of the species identified in this study. USGS/Public Domain.