Conservationists often object to wind farms because of the possibility that they could kill birds. But birds aren’t the only flying animals to be taken out by turbines – it turns out that bats often lose their lives too, and not in quite the way you might imagine.
Recently, scientists have noticed a large number of dead bats at wind farms around the world. The turbines seem to be taking a particularly heavy toll on migratory species and while it was clear that the scale of these deaths is much larger than expected, it’s less clear why they’re happening at all.
Bats can fly through pitch-darkness through echolocation – a superb biological sonar system that’s particularly good at detecting moving objects. With such state-of-the-art equipment, it’s difficult to believe that these accomplished aeronauts could simply be blundering into the spinning blades. Now, Erin Baerwald and colleagues at the University of Calgary think that they have found the mystery killer, and it’s one that is invisible to the bats’ sonar – areas of low pressure.
As the blades of wind turbines spin, they reduce the pressure of the air around them. As bats fly into these low-pressure zones, the sudden change causes the air in their lungs to expand faster than the bats can exhale. The technical term is “pulmonary barotrauma”. In three syllables, “their lungs burst”. Areas of low pressure are essentially invisible – even if the bats’ sophisticated sonar guides them through the spinning blades, they wouldn’t be able to sense these danger zones until it’s too late.
CSI Alberta
Baerwald performed autopsies on 188 hoary and silver-haired bats recovered from a wind farm in Alberta, Canada, to determine their cause of death. She found that 90% of them had some form of internal bleeding in their chest cavity that is consistent with death by decompression. A thorough dissection confirmed Baerwald’s interpretation – the majority had damaged lungs and some had distinct air-filled bubbles that were formed by the rupture of their alveolae (the tiny sacs in the lungs through which gases are exchanged with the blood).
It’s not clear what extent of pressure drop would be fatal to a flying bat, but based on the speed of modern turbines, Baerwald calculated that their tips create a fall in pressure of 5-10 kPA (kilopascals). That’s more than enough to seriously harm a lot of mammals and rats cannot withstand even a drop of 4.4 kPa.
About a third of the bats killed by barotraumas also suffered from external injuries, picked up by hitting the blades themselves. It’s likely that these animals stumbled into the turbines after suffering from internal injuries, for only 8% of them – just 6 animals in total – had died from external injuries with no accompanying internal ones.
Birds and bats
It’s possible that the scale of bat fatalities has simply been underestimated in previous studies, which were primarily designed to look for dead birds. Birds are less often killed by wind turbines and Baerwald thinks that their unique breathing apparatus may make them less vulnerable to sudden pressure drops than those of bats. Bat flight is fuelled by large lungs and hearts, oxygen-rich blood and an unusually thin layer separating the gas in their lungs from the blood in their vessels.
In contrast, the lungs of birds are more rigid. Unlike the balloon-like sacs of mammals, those of birds are tube-like, compact and have exceptionally strong capillaries. In low-pressure zones that would pop the lungs of bats, those of birds barely expand. It’s a case of the robin outclassing the bat. Birds do however have a relative disadvantage; they lack the sonar of bats (most species anyway), so most of their turbine-related fatalities are caused by direct hits.
As wind turbines become more common, one group of scientists predict that by 2020, they will kill anywhere from 33,000 to 110,000 bats per year. That isn’t a reason to cease the development of wind technology, but it is a reason to think about ways of protecting passing bats. Situating wind farms away from migration routes is one possible solution, although these routes aren’t exactly thoroughly mapped out. Shut-off times for turbines during periods of high bat activity is another, although that would slash their electricity production.
Whatever the solution, it’s heartening to see that the issue is at least being taken seriously. After all Baerwald’s research has an intriguing blend of funders – three conservation organisations, such as Bat Conservation International and five energy providers, including Shell Canada.
Reference: Current Biology 18: R695
August 28th, 2008 at 10:21 am
Part of the reason they’re flying through the low pressure zones caused by the passing fan blade is that they cant see it, they use sonar. This allows them to fly incredibly close to the fans with confidence. Use the sonar which steers them close to the fans to steer them away from it. Either some sort of sonic pulse which is sent at a high frequency to annoy the bats (but not mess up their echolocation) or some other form of sound which would keep them away from the fans.
August 28th, 2008 at 10:25 am
Bats cannot ‘see’ a rapidly moving object that comes out from hiding, which is why it’s easy to hit a bat with a long stick.
August 28th, 2008 at 10:29 am
“it’s easy to hit a bat with a long stick.”
A long stick. You mean a bat?
It’s disturbing that you know this.
August 28th, 2008 at 11:08 am
“some sort of sonic pulse which is sent at a high frequency to annoy the bats”
A sonic pulse. You mean a bat signal? (This is too easy – I’m stopping now).
August 28th, 2008 at 1:48 pm
Keep up the puns and you might get battered.
August 28th, 2008 at 5:42 pm
Fortunately, it has also been discovered that bats avoid radar stations. There are already suggestions to scare bats away from wind farms with radar systems (citation: http://www.plosone.org/article/info:doi%2F10.1371%2Fjournal.pone.0000297 ).
August 28th, 2008 at 5:43 pm
I love to watch bats as they come out at night, so the windmills-killing-bats stories always attract my attention. It’s good to know that the reason had been found. Now I can lay my rather idiotic armchair speculations about small internal parts looking like bugs to rest.
Also, it seems unfortunate that the new data doesn’t seem to give any new ideas about how to mitigate it. Keep us posted on that.
August 29th, 2008 at 4:51 am
Easy problem to fix, just make some sort of wind turbine out of bats themselves, I’m not much of a scietnist but blue peter has taught me a little bit of cellotape and PVC glue goes along way, if any biotech company wants to employ me, I can be found at this address;
Dr Bunsen….and assistant beaker
Sky Labs
Sesame street
August 29th, 2008 at 10:09 am
Ah – some friends have several bat colonies in and around their home (bats in their attic for real). They are worried about proposals for a windfarm near their home. But, if this affects migrating bats more than bats out and about on their normal flight-paths, perhaps this might reassure them on this point.
August 29th, 2008 at 2:01 pm
This is, ultimately, an engineering problem. There will be lots of ways to address it that will work, more or less, at various costs, and one of them will turn out to be the best. Each person advocating one of the other alternatives will be disappointed, but will have lots of company.
August 29th, 2008 at 2:15 pm
Well said Nate. Oh, the unintended consequences of cleaner energy. Next up, “ambient heat from dark colored solar panels attract endangered insect, leading to DEATH!”
August 29th, 2008 at 11:35 pm
The added bonus of sa3r3t’s suggestions of “some sort of sonic pulse which is sent at a high frequency to annoy the bats” is that it might also keep away the kids…
http://sciencevideos.wordpress.com/2008/02/19/the-mosquito-hoodie-repellent-from-the-valleys/
August 30th, 2008 at 8:42 am
Isn’t it possible to fool the bat’s sonar to think the wind farm is just a big solid object and so make them avoid it?
August 30th, 2008 at 9:32 pm
Now that they understand this, they should be able to develop some kind of ultrasonic signal that repels bats, since the echo-location functions of bats are so sensitive to certain frequencies
September 1st, 2008 at 8:46 am
OK, look at a desktop fan and aplly the same … place a grid around it .. !!
September 3rd, 2008 at 12:22 pm
Let’s not panic about a single study of a single wind farm that included only 188 bats. Have other possible causes for the barotrauma been ruled out? How many experts were consulted on possible causes. I am not dismissing the importance of evaluating environmental impacts, on the contrary, wind power has possibly enormous environmental benefits. Basing widespread policy decisions on a single, geographically limited study is not good policy or judgment.