Bats have been a misunderstood and widely persecuted species for almost long as there has been a recorded human history. As a result, bat mortality at wind energy facilities was a largely unanticipated phenomenon.
However as wind farms are built with increasing frequency, bat deaths at these facilities is becoming a more prominent concern. This is particularly true in the US, Canada and Europe where wind energy facilities have been associated with an unprecedented rise in bat deaths.
While it was once thought that the wildlife impacts of wind energy facilities would primarily effect birds, in recent years researchers have found that bat fatalities significantly outnumber bird deaths at most wind energy facilities studied.
The latest to delve into the phenomenon are researchers from the University of Wisconsin-Madison in the US.
The team, which conducted their study at the Forward Wind Energy Center in southeastern Wisconsin over a two-year period, was led by UW-Madison forest and wildlife ecology professor David Drake and graduate student Steven Grodsky.
They partnered with Melissa J. Behr and others at the UW-Madison School of Veterinary Medicine and Wisconsin Veterinary Diagnostic Laboratory to conduct environmental assessments of the facility and post-mortems on bats sporadically found dead on the facility grounds.
The research project was funded by Invenergy, a renewable energy company and Wisconsin Focus on Energy.
Going into their work, the researchers knew that there were two previously identified causes of death of bats at wind farms. Once, of course, is blunt-force trauma cause by colliding with turbine blades. The other is barotraumas, which is caused when bats fly through areas of dramatically different pressure created by spinning turbine blades.
Previous studies had surmised that of the two, barotraumas is the leading cause of death of bats traversing wind farms, but in the interview that follows, David Drake said his team came away unconvinced, and suggested that even after using advanced veterinary diagnostic techniques, X-rays, and full-blown necropsies of the victims, it was still difficult in one, the other or both phenomenon simultaneously played a role in the deaths.
“Nearly 75 percent of the bats had broken bones, mainly in the wings, and the majority had sustained a mix of skeletal fractures and soft-tissue damage such as ruptured organs, internal bleeding, and hernias,” Drake said. “Roughly half of the bats examined also had middle and/or inner eardrum ruptures.”
The team’s work has been published in the Journal of Mammalogy.
You’ve been doing research on bats and bat populations for some time now and tackled a lot of different issues; What lead you to look at wind turbine-related bat mortality?
I was a faculty member at Rutgers University in New Jersey before I cam here to the University of Wisconsin, and toward the end of my time at Rutgers the state public service commission asked me to do a comprehensive literature review of wind energy relative to wildlife, and specifically flying wildlife, birds and bats.
As I was putting that report together, it dawned on my that there really isn’t a lot out there about this particular topic, and a lot of what has been published has not been published in the scientific, peer-reviewed literature, but more often in what are called “gray reports” that are done by environmental consulting firms and never really see the light of day.
So when I applied for a job at the University of Wisconsin, one of the things I needed to figure out was what my research focus would be; at the time, I was getting more and more into this line of research related to wind energy and Wisconsin itself was just entering the era of wind energy development in the state, so I thought it would be an interesting area of research.
All of the research that was covered in this article was out of one particular wind farm in the southeast section of Wisconsin.
Why that wind farm? Had they experienced a rash of bat and/or bird deaths? Was it a matter of proximity to the university?
Well, let me provide a little more context. The permitting of wind farms is different in every state, but in many of them, the permitting process is handled through the state’s public service commission, a regulatory agency. In Wisconsin’s case, any wind farm over 100 MW has to be permitted by the public service commission and a requirement of that is that they do a two-year, post-construction study on bird and bat mortality. So we were contracted to do that study by Ford Energy.
We did two seasons of fall migration and two seasons of spring migration, so we essentially monitored mortality over the course of two years.
So when you went out to do the field-work for the study, where you regularly finding dead birds and bats?
It was really sporadic, actually. Just to give you a sense, over the course of the two years we found a total of 20 birds. And we found about 140 or 150 bats. Most of the bat mortality occurs during the fall migration period, and there are a couple of peaks in when this mortality occurs that fall within the last week of August and into the first couple of weeks of September.
That said, we do get bat mortality in the spring and at other times during the fall migration. It’s just not in those concentrations that we see during the peak period. The other thing I should point out is that it’s not as if bats are just raining down out of the sky. I think at most we found three or four bats in one day. Most of the time you just find a single bat, here or there, per day.
Do bats migrate like birds do?
It depends on what the species is. Here in Wisconsin, we have eight species of bats, but one of those species is the federally-endangered Indiana bat, and that species, for one thing, has not been definitively seen in Wisconsin in decades, but also when it was seen, it was only in the extreme southern portion of the state – so its historic range was not in the region of this wind farm where we did our work.
So that leaves us with seven bats species. Three of these species are tree-roosting bats – the eastern red bat, the hoary bat and the silver-haired bat – and those are true migratory bats that leave our state in the fall and come back up in the spring.
And then we’ve got four bat species – the little brown bat, the big brown bat, the eastern pipistrelle and long-eared myotis – that are resident species and usually hibernate in Wisconsin in the winter, although there is some thought now that there might be some short-distance migration as well. They might move from northern Wisconsin to southern Wisconsin or from Wisconsin to Illinois or something like that, but they are not true migratory bats, despite exhibiting some of that migratory behavior.
One of the phrases used in the report that I’m unfamiliar with is barotrauma. What is barotrauma?
Probably the best analogy is when a human scuba diver gets the bends from coming up too quickly from the depths. Essentially what happens is the bat’s internal organs implode because of extremes in air pressure.
Bats can fly through the rotor-swept zone successfully. Even though the blades are turning, they can still get through there without getting hit, but there is a pressure differential on the front of the blades to the back of the blades, and also a pressure differential surrounding the tips of the blades. So as the bats fly through these pressure differentials, which can be pretty drastic in terms of pressure gradient, their skeletal and anatomical features are not strong enough to hold that body together.
So their internal organs are essentially being ripped apart was they go from these extremes of high pressure to low pressure. Again, it’s very similar to the bends in humans, which in extreme cases can cause the organs to hemorrhage.
That’s something I never really considered. I guess when I think of wind turbine-, or wind blade-related mortality, I always think of something being hit by the blade. And by extension, I would have thought that bats might be less likely to be in this situation because they fly and navigate with the aid of sonar…
Well, let me tell you a couple of things that came out of our study. In 2008, Erin F. Baerwald and Robert M.R. Barclay of the Department of Biological Sciences, University of Calgary in Alberta, Canada presented a paper that suggested that barotrauma was the primary cause of [wind-farm related deaths] of bats and a lot of times when you do mortality searches at wind turbines, you find bats closer to the turbines and birds further away from the turbines. This makes some sense because birds can fly through the pressure differentials and because of their skeletal features and anatomy, they are better able to withstand that pressure gradient. So most of the birds we found seemed to have been killed by blunt force trauma, and they seemed to have either been able to fly on or to have gotten flung a little bit further than the bats did.
With bats, if the barotrauma affects them, they are killed instantly, and so it makes sense that they would fall closer to the turbine.
So in 2008, Baerwald and Barclay came out with this paper suggesting that barotraumas was the primary cause of death for bats at wind farms, and they had done some field necropsies, or autopsies on the bats, to determine that.
I had a graduate student Steve Grodsky, who is the lead author of the paper, and I said, “Steve, why don’t we look into this barotraumas hypothesis and either support it or refute it?” So Steve went over to the University of Wisconsin veterinary medical school and the Wisconsin veterinary diagnostic lab, and teamed up with some veterinary pathologists over there. Then we took a sub-set of the bats Steve was collecting, we did X-rays on them, histology and tissues samples, and we did a very extensive necropsy on them in a clinical setting.
In doing that, we found that we were not able to determine a clear cut cause of death – whether the death was caused by barotraumas or blunt force trauma. Instead we found both. In most of the bats – about three-fourths – we found broken bones and we also found hemorrhaging in the chest cavity, the abdominal cavity and certainly in the head region as well. The barotraumas would affect primarily the organs in the chest and the abdominal cavity. But you also get hemorrhaging from blunt force trauma as well.
So unlike Baerwald and Barclay, we did not find that one cause of death was more prevalent than the other. We think that these deaths are due to both. We think bats are dying from both barotraumas and blunt force trauma and in most cases it is very likely that the bat is suffering blunt force trauma and barotraumas simultaneously.
Now to get to the issue you mentioned, echo-location, one of the authors of the study is a physicist, and he came up with calculations that suggested that if a turbine blade is spinning at roughly 170 metres per second – and assuming the reach of a bat’s echo-location is about 60 metres – a bat would have roughly one-quarter of one second to react to the blade.
That’s why when people say, “Gee, these turbines are standing up in the agricultural landscape, why can’t the bats see them?” the answer is, they probably do see them from a distance, but as they get closer to them, they probably don’t have enough time to react to the spinning blades.
Okay, now it seems, from a layman’s perspective, that there’s a margin of error that’s unavoidable with this kind of study, and that’s because there’s the phenomena of delayed mortality. I know you address this in the report. How did you deal with it? How do you know what ultimately happens to bats that are seriously injured and perhaps don’t die right away?
You don’t. That’s the problem. One of the things that goes on with these mortality studies is you have all of these researchers who go out and pick up all these dead bats and dead birds and that’s your raw number of carcasses found, but then you also have to monitor and measure searcher efficiency and scavenger removal.
The way you do that is you go out, and unbeknownst to the researchers, you put out X number of bat and bird carcasses and test the searchers in terms of how many they found or didn’t find. That factors into their searcher efficiency rate. Then you’ll put out carcasses across the landscape, separate from the searcher efficiency carcasses, and you’ll leave them out for a certain amount of time, come back, and determine how many are left, and that will give you your scavenger removal rate. You then plug this information into a mathematical formula to come up with a corrected mortality estimate.
What we are suggesting when we discuss sub-lethal effects – and specifically we talk about ear damage, which we fond in about half the bats we recovered, where the ear membrane had been ruptured – and when that happens that effects the bat’s balance and their flight capability, and if they can’t fly very well, then they can’t move across the landscape or feed and hunt for food, so ultimately that’s going to kill them, but they may not die within our search area.
That’s what in our study we call these sub-lethal effects, especially when it comes to inner ear trauma. The bat still may be alive, but eventually it is going to perish. So we are suggesting that it is possible that these bat surveys are underestimating the rate of bat mortality because there might be more sub-lethal effects going on that we realize.
Now that the study is done, how do people act on your findings? Can they act to these findings? Is there anything that can be done to reduce the mortality rate?
That is a very, very good question and unfortunately, it’s a very hard question to answer. Part of the issue is this: All of these wind farms exist to make renewable energy; some are publicly-owned, some are privately-owned; but they all have to make a profit. So we are trying to balance the issue of renewable energy and the production of renewable energy, with negative impacts such as the mortality or birds and bats.
As a result, you have to be careful when you propose any mitigation strategy that it is a cost-effective mitigation strategy. It can’t be so expensive that it puts these wind companies out of business. So that’s the $64,000 question. We now have a better understanding of how exactly the bats die. But we still don’t know why these bats are attracted to the wind farms in general. Are they thee just randomly and would be there whether the wind farm was there or not? Or, are the turbines attracting them for some reason and if the wind farm wasn’t there the bats might not be there?
That’s the question we are still trying to figure out and that is a very, very important question. But now that we know that barotraumas is an issue, perhaps something can be done in the future design of a turbine blade – and I’m not an engineer and don’t even know if this is feasible – that would reduce the pressure gradient, so you don’t have such a great pressure difference there or that the pressure difference is more gradual than it is now, so that if a bat were to fly through it, they would sustain any grave internal damage.
The issue with blunt force trauma is a far more difficult issue to get around because ultimately as long as turbines are spinning, it is likely that they are going to collide with bats and the bats are going to collide with them.
Another paper that was done by Baerwald and Barclay in the last year or so, looked at turning the turbines off on nights when they suspected there would be a lot of bat migration, and I can’t remember what the exact number was, but they reduced bat mortality by a significant percentage just by turning the turbines off on those select nights.
What about a repellant of some sort? Might there be some way to dissuade bats from flying across a field dotted with wind turbines?
Well, I don’t know if you’re familiar with the technology, but there’s something called an Anabat, which is an ultra-sonic bat recorder… in other words, a passive listening device. So when bats emit this ultra-sonic call, this recorder picks it up, and that’s one of the ways you are able to record bat activity.
Some people are thinking about trying to reverse that, and instead of recording bat calls, they are trying to send bat calls out. Now, bear in mind, this hasn’t been tried yet, but perhaps that would be away to alert them and say, “Hey, don’t come into this area; there’s danger here.”
Starlings, the bird, when they are not feeling well or are in danger, they emit this kind of distress call to other Starlings warning them to stay away. So there’s a school of thought that says, couldn’t something be done with these Anabats, but to my knowledge nothing has been tried and nothing has been worked out in the field to even see if that is feasible.
Photo caption: Radiograph of a bat killed by a wind turbine at a southeastern Wisconsin wind energy facility during fall 2009. Note the compound fracture of the right arm and the dislocation of the right shoulder joint. Photo provided by David Drake.
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