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Wind Energy: A Scare for Bats and Birds

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Several USGS scientists are investigating the problem of fatal bat and bird collisions with wind turbines. USGS scientist and bat specialist Dr. Paul Cryan at the Fort Collins Science Center chats with Juliette Wilson about whether we can have our wind turbines and healthy populations of bats and birds too.




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Juliette Wilson: Hello and welcome the USGS CoreCast. I’m Juliette Wilson at the Fort Collins Science Center in Colorado. As a nation, we are experiencing a welcome surge in the development and use of green energy sources, but at least one of these is having unintended consequences.

Wind farms have been associated with significant bird and bat deaths from collisions with the big turbine blades. Certain bats in particular seem most vulnerable. Several USGS scientists are investigating the problem of fatal bird and bat collisions with wind turbines. To help understand the problem and to find out if we can have our wind turbines and healthy bats and birds too, we turn to Dr. Paul Cryan, a research ecologist and bat specialist here at the Fort Collins Science Center.

Welcome Paul and thanks for being with us today.

Paul Cryan: Thanks Juliette. A pleasure to be here.

Juliette Wilson: So Paul, what can you tell us about the situation with bat mortality and wind turbines? What’s happening out there?


Paul Cryan: Well, it all came at us from out of the dark. Back in the late 1990s, we started hearing reports from biologists studying birds at wind energy sites that they’re finding dead bats beneath wind turbines. Those reports trickled in until in 2003, there was an incident in the Appalachian mountains of the eastern US where a few hundred dead bats were found beneath turbines at a site within about a 6-week period and that really got a lot of us wondering what’s happening with the situation and ever since then we’ve been scrambling to try to find out what’s going on and come up with solutions to the problem.

Juliette Wilson: Well Paul, what’s killing the bats? Are they actually colliding with the fast moving blades or is it something else?

Paul Cryan: Yes. Well, yes and there is something else. We know that they are hit by the blades. The blade tips of a lot of these turbines are moving over a hundred miles per hour. So, we see a lot of fatalities that have traumatic injuries that tell us that they were hit. But we also know that bats are also susceptible to what we call barotrauma.


They’re being exposed to pressure changes associated with the spinning blades that as they come into close contact with them, they may not even have to be hit by a blade, but that pressure drop is so quick and rapid that when a blade goes past it, their lungs fail. So yes, they are being hit and they’re also having these close contact exposures that are fatal.

Juliette Wilson: Which bats are most affected?

Paul Cryan: Well, we have 45 species of bats in the US and Canada and three species that compose the majority of wind turbine fatalities so far. More than 75% are the hoary bat, the eastern red bat and the silver-haired bat.


Estimated fatality rates near range between about one bat per megawatt per year to over 50 bats per megawatt per year. And those numbers probably don’t mean a lot of to most people so I’ll put them in perspective. The safest sites were probably killing dozens of these bats per year, but some of the more risky sites are probably killing thousands per year.

So, already in North America, we may be ranging into the tens of thousands of bats. We don’t really have good numbers yet, but those are pretty reasonable estimates I think.

Juliette Wilson: What about birds? Are you seeing the same levels of fatalities with birds?

Paul Cryan: Birds are being killed by wind turbines at some sites in particular, but I don’t think at anywhere near the magnitude that migratory tree bats are being affected. Another striking difference between bird and bat fatalities is that dozens of species comprise the overall fatality rates of birds whereas with bats, as we've talked about proportionally few species make up most of the fatalities.


Hoary bats for example, have been found dead beneath wind turbines at every wind facility that’s been adequately monitored for bats. And I haven’t heard about any bird specie suffering such consistent losses across the continent for example. So, we think something awful is happening with bats at turbines.

And it’s probably some kind of a uniquely unfortunate aspect of their behavior that may even draw them into wind turbines and fatal attraction possibly.

Juliette Wilson: What are you and others doing to evaluate the problem? Why is it happening?

Paul Cryan: Well, there’s a lot of us working on many different fronts of this issue. I’m personally involved in research, trying to figure out why the migratory tree bats in particular are so susceptible. Looking into the causes of fatality. Why are they in the vicinity of the turbines in the first place? Is their something about their behaviors that may put them at greater risk?


Whether it’s migration, whether it’s mating behavior. And some of the research I’m doing right now looks at those two aspects. Other people are working hard to come up with ways to try to assess risk of a turbine to bats before they’re built. So, wind energy companies and resource managers would like to know will these turbines that are proposed for the land under our care pose a risk to bats if we build them.

Right now, we don’t have a very clear way of doing that but people are working towards those kinds of risk assessments method. And a lot of the people are also working on solutions to the problem, or ways to mitigate it and that’s the promising area of research too.

Juliette Wilson: So, what solutions are possible? Are any available at this time?


Paul Cryan: We’re close. There’s a very promising mitigation technique on the horizon. It’s been tested in Europe, Canada and the US now. And the pattern we see with bats fatality is that turbines is that most of the fatalities occur at night during this period of late summer and fall and during the low wind conditions. And these experiments have shown that keeping the turbine blades from rotating under low wind conditions during that narrow seasonal window at night, that all the sites found an effect of 50% fatality reduction. So, that’s a very promising technique that still needs to be researched under wider conditions, but that’s the hope that we have right now.

Juliette Wilson: Well, great. Thanks so much for being here today Paul.

Paul Cryan: Thank you. It’s been a pleasure.


Juliette Wilson: For more information on bats and wind turbines, we invite listeners to visit where you will find pictures, animated seasonal distribution maps and more information. Again that Internet address is

CoreCast is a product of the U.S. Geological Survey, Department of the Interior.

Announcer: Juliette Wilson is an Information Management Specialist with ASRC Management Services under contract to the USGS.

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