Wind turbines are an important source of clean, renewable energy. They're one of the fastest-growing power sources in the U.S., outpacing even natural gas. Unfortunately, they also sometimes kill birds and bats.

That may sound like an environmental Catch-22, but it doesn't need to be. From new designs and smarter locations to high-tech tracking systems and ultrasonic "boom boxes," many American wind farms are experimenting with various ways to make their turbines safer for flying wildlife.

Wind turbines were never the top threat for most birds. A study published in the journal Biological Conservation found that U.S. turbines kill 234,000 birds per year on average, while a newer study, published in Energy Science, found that about 150,000 birds are affected by wind turbines in the U.S. per year. By comparison, research suggests up to 1 billion U.S. birds die each year after colliding with windows, and up to 4 billion more are killed by feral cats. Other threats include high-tension wires (174 million birds), pesticides (72 million) and cars (60 million).

And perhaps the No. 1 threat to birds is climate change, which is driven by the very fossil fuels wind turbines are meant to displace. According to a report by the National Audubon Society, two-thirds of America's birds are now threatened with extinction due to climate change, especially Arctic birds, forest birds and waterbirds.

As for bats, wind farms may also pose a different kind of risk. When a bat flies into a patch of air immediately after a blade tip has passed by, the sudden drop in pressure can reportedly rupture its lungs, a condition known as "barotrauma." Research is mixed on this subject, though, with a 2008 study calling barotrauma a "significant cause of bat fatalities" and a 2013 study arguing blade strikes are a more likely culprit. Either way, roughly 600,000 bats die on U.S. wind farms per year.

hoary bat, Aeorestes cinereus Hoary bats are among the bat species most commonly harmed by wind turbines in the U.S. (Photo: Michael Durham [CC BY-SA 2.0]/Oregon State University/Flickr)

That's a real problem, but not on the scale of white-nose syndrome, a deadly fungal disease that has spread from one New York cave in 2006 to at least 33 U.S. states and seven Canadian provinces. With a mortality rate as high as 100% and no known cure, it poses an existential threat to some entire bat species, especially if they're already endangered by things like pesticides or habitat loss.

Nonetheless, wind farms still kill too many bats and birds overall. These losses can compound the animals' other woes, and they also undermine the role of wind as an environmentally beneficial power source. On top of directly helping today's birds and bats, solving this could indirectly help everyone on Earth by boosting the case for wind farms versus older energy sources that fuel climate change.

To that end, here are a few ideas that might help wind farms coexist with birds and bats:

1. Safer locations

white-tailed eagle flying in Hokkaido, Japan Wind turbines may pose less danger to raptors if they're sited away from cliffs and hills where the birds of prey seek updrafts. (Photo: Jiri Hrebicek/Shutterstock)

The simplest way to keep birds and bats away from wind turbines is to not build wind turbines where lots of birds and bats are known to fly. It's not always that simple, though, since many of the open, treeless expanses that attract birds and bats are also prime locations for harvesting wind.

Already-altered habitats like food farms make good turbine sites from a wildlife perspective, according to the American Bird Conservancy, but the main thing to avoid is any habitat deemed an "Important Bird Area." These include places where birds congregate for feeding and breeding, like wetlands and ridge edges, as well as migratory bottlenecks and flight paths used by endangered or declining species.

In the aforementioned Energy Science study, researchers found "no significant impact" from wind turbines as long as they were located 1,600 meters (about 1 mile) away from high-density bird habitats. "We found that there was a negative impact of three birds lost for every turbine within 400 meters of a bird habitat," says study co-author Madhu Khanna, professor of agricultural and consumer economics at the University of Illinois, in a statement. "The impact faded away as distance increased."

While more than 60% of all avian deaths at U.S. wind farms are small songbirds, they account for less than 0.02% of their total population, even for the hardest-hit species. Still, although wind turbines may be unlikely to cause population declines for most bird species, the American Wind Wildlife Institute has warned that "as many species decline because of a host of other factors, the potential for biologically significant impacts to some species, such as raptors, may increase." To help, developers can locate turbines away from cliffs and hills where raptors seek updrafts.

Environmental assessments are now a key part of planning new wind farms, often using mist nets, acoustic detectors and other tactics to assess bird and bat activity before deciding on turbine sites.

2. Ultrasonic 'boom boxes'

ultrasonic 'boom box' for protecting bats from wind turbines An ultrasonic deterrent device, aka 'boom box,' used to repel bats from wind turbines. (Photo: E. Arnett [CC0 1.0]/Bat Conservation International/PLOS One)

Birds are mostly visual animals, but since bats use echolocation to navigate, sound might offer a way to repel them from wind farms. That's the idea behind ultrasonic "boom boxes," which can be attached to turbines and emit continuous, high-frequency sounds between 20 and 100 kilohertz.

Bats' sonar is good enough to work around such interference, researchers reported in a 2013 study, but it might still be enough of a hassle to keep them away. "Bats can actually adjust their echolocation under jamming conditions," they wrote. "Bats are, however, likely 'uncomfortable' when broadband ultrasound is present because it forces them to shift their call frequencies to avoid overlap, which in turn will lead to suboptimal use of echolocation or they may not echolocate at all." Between 21% and 51% fewer bats were killed by boom-box turbines than by turbines without the device, the study's authors added, although some technical hurdles remain before the technique has widespread practical value.

"Our findings suggest broadband ultrasound broadcasts may reduce bat fatalities by discouraging bats from approaching sound sources," they wrote. "However, effectiveness of ultrasonic deterrents is limited by distance and area ultrasound can be broadcast, in part due to rapid attenuation in humid conditions."

3. New colors

wind turbine silhouettes at twilight Researchers have studied using paint and UV light to deter birds and bats from flying near wind turbines. (Photo: kamilpetran/Shutterstock)

Most wind turbines are painted white or gray, an attempt to make them as visually inconspicuous as possible. But white paint can indirectly lure birds and bats, researchers found in a 2010 study, by attracting the winged insects they hunt. White and gray turbines were second only to yellow ones in attracting insects, according to the study, including flies, moths, butterflies and beetles.

Purple turned out to be the least attractive color to these insects, raising the possibility that painting wind turbines purple might alleviate some bird and bat fatalities. The researchers stopped short of advocating that, however, noting that other factors — such as heat given off by turbines — could also be encouraging wildlife to fly near the spinning blades.

Even if purple paint isn't practical, another line of research is investigating the use of ultraviolet light to deter birds and bats from turbines. While UV light is invisible to humans, many other species can see it — including bats, which aren't as blind as you might have heard. Still, given the limitations of long-distance vision at night, some researchers think migrating bats don't always see the spinning blades, and mistake the poles of wind turbines for trees. Rather than trying to deter bats at short range, a team of researchers with the U.S. Geological Survey and the University of Hawaii are studying how dim UV lights on turbines can warn bats about the danger from afar, broadcasting "this is scary" to bats before they get too close.

4. New designs

Beyond new paint and scary lights, tweaking the structure of wind turbines could greatly reduce the risk they pose to birds and bats. Engineers have come up with a wide array of wildlife-friendly designs in recent years, ranging from slight modifications to overhauls that barely resemble a traditional wind turbine.

In the Energy Science study, researchers found that the size of the turbine and the length of the blades can make a substantial difference. Just making the turbines taller and the blades shorter reduces the impact on birds, the study's authors report. In addition to regulating the location of turbines, they suggest, wind-energy policies should promote greater turbine heights and shorter blades to protect birds.

And then there are the more dramatic reinventions. A concept known as Windstalk, for example, doesn't even use spinning blades. Developed by New York design firm Atelier DNA, it's meant to harness wind energy with giant, cattail-like poles that mimic "the wind sways a field of wheat, or reeds in a marsh." Other alternatives include vertical-axis turbines, sail-like wind dams, high-flying energy kites and a helium-filled blimp that would fly 1,000 feet high, placing it above most birds and bats.

5. Radar and GPS

bat on radar map in Texas A gathering of bats appears on a radar image from central Texas. (Image: U.S. National Weather Service)

Weather radar often picks up more than weather. In the image above, for example, National Weather Service radar detected a huge crowd of bats flying at sunset over central Texas in June 2009. If wind farms have quick access to high-quality radar images like those, they could shut off their turbines to let flocks fly through.

Identifying animals from radar isn't always easy, especially for small bats and songbirds, but it's getting better. The best use of radar might be prevention, helping us avoid building wind turbines in places where birds and bats tend to congregate, but it can also help existing wind farms make life-saving adjustments. In Texas, some coastal wind farms have used radar for years to protect migrating birds. And there are products available like the MERLIN avian radar system, made by Florida-based DeTect, which scans the skies for 3 to 8 miles around wind-energy sites, both for "pre-construction mortality risk projections and for operational mitigation."

For especially endangered species like California condors, GPS can provide an extra level of protection. Although it wouldn't work for most species, about 230 California condors are outfitted with GPS transmitters that allow nearby wind farms to keep track of their whereabouts.

6. Restraint

flock of birds flying near wind turbine The operation of wind turbines can be adjusted in several ways to create less danger for birds and bats. (Photo: Bildagentur Zoonar GmbH/Shutterstock)

Researchers from Oregon State University are developing sensors that can tell when something hits a wind turbine blade, giving operators a chance to prevent more collisions by shutting turbines down. Along with those sensors — which researchers are testing by launching tennis balls at turbine blades — cameras could be mounted on turbines to show operators if birds or bats really are in the area.

Before anything hits the fan, however, wind-farm operators also have other options beyond radar to anticipate the arrival of flying wildlife. Most bat fatalities occur in late summer and early fall, for example, when many species of bats are most active. Bird migrations also tend to follow seasonal patterns, giving wind-farm managers a chance to shut down their turbines before the largest flocks try to fly through.

Bats also typically prefer to fly in weak winds, so leaving turbines dormant at lower wind speeds — known as raising the "cut-in speed" at which they begin generating power — can save lives, too. In one study, published in the journal BioOne Complete, researchers found that leaving turbines idle until winds reach 5.5 meters per second curbed bat deaths by 60%. And another study, published in Frontiers in Ecology and the Environment, found that bat mortality was up to 5.4 times higher at wind farms with fully operational turbines than at those with reduced activity. Raising cut-in speeds is more expensive for electric companies, the researchers acknowledge, but the lost power is less than 1% of total annual output — a low price to pay if it can prevent mass wildlife casualties.

"Relatively small changes to wind-turbine operation resulted in nightly reductions in bat mortality, ranging from 44% to 93%, with marginal annual power loss," they wrote. "Our findings suggest that increasing turbine cut-in speeds at wind facilities in areas of conservation concern during times when active bats may be at particular risk from turbines could mitigate this detrimental aspect of wind-energy generation."

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Wind turbines will likely always pose some degree of risk to wildlife, as do cars, airplanes and many other large, fast-moving machines. But as more wind farms heed ecology and apply better technology, the risk is shrinking enough to unite conservationists and wind-energy advocates against a common foe: climate change. And in a sign of that unity, the U.K.'s Royal Society for the Protection of Birds offered an olive branch in 2016 by building a wind turbine in a field next to its headquarters.

"We can already see the impact that climate change is having on our countryside," the RSPB's Paul Forecast said in a statement when the plan was announced. "It is our responsibility to protect the rest of our environment for future generations. We hope that by installing a wind turbine at our U.K. headquarters, we will demonstrate to others that, with a thorough environmental assessment, the correct planning and location, renewable energy and a healthy, thriving environment can go hand in hand."

Editor's note: This article has been updated with new information since it was originally published in October 2014.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

6 ways to protect bats and birds from wind turbines
Wind turbines can kill birds and bats, but they don't have to. Here are a few ways to help them coexist.