Weekend Reads | Reducing Bird Deaths on Wind Farms

by Kevin Schofield

This weekend's read looks at one way to prevent wind turbines from killing birds. It's been well-documented for many years that birds do sometimes collide with the blades of wind turbines. While some people have invoked this as an argument against building wind farms, the data suggests that house cats kill more birds than wind turbines, and that fossil-fuel-based energy generation probably also causes more bird deaths.

That said, it's important to understand whether anything can be done to prevent bird collisions with wind turbines. That turns out to be a complicated question, particularly since there are ongoing disagreements about why a bird might collide with a wind turbine blade. Some have suggested that some hunting birds, such as raptors, are so focused on tracking prey on the ground that they don't pay attention to moving objects at their height; this would explain why raptors are over-represented among the birds found dead on wind farms. But there is another hypothesis: that birds' vision system doesn't allow them to clearly see the moving blades when they are close by.

A great research paper from 2003 lays out the physiology behind the problem. The retinas of our eyes can only send signals to our brains at a certain rate; but instead of sending a snapshot of what they see at a particular time, they average what they see over the previous 120 milliseconds — just over a tenth of a second. The resulting effect is called "motion blur" or sometimes "motion smear": things moving too fast for our eyes to get a crisp picture of them just look like a blur. Movies and television use this to their advantage: Standard movie film projectors show 24 frames per second, and our eyes blend the frames together so that we see continuous motion instead of individual frames. This is also why when you look at a fan that's running, it looks motionless and semi-transparent: our eyes are averaging together the time when a fan blade is in front of a particular spot and the time when it isn't. Likewise, if you look out the side window of a car and another car whizzes by you in the next lane, it's a blur; but a car moving the same speed farther away is clearer because the farther-away one is moving more slowly across your eye. Generally, the closer a moving object is to you, the more blurry it will look.

Birds' vision works the same way. Theirs is better than ours — especially for raptors — and their retinas may react faster than 120 milliseconds. But at some point, they still encounter the same problem: motion blur for objects moving too fast across their field of vision. The prevailing theory now for why birds run into wind turbines is that once they get close enough to a blade, the motion blur prevents them from being able to see it.

This has led scientists to look at whether there are changes they can make to how wind turbines are manufactured to reduce motion blur. They have looked at the shape of the blades; the color of the blades; and even painting patterns on the blades.

Changing the shape of the blades is complicated, because wind turbine blades are shaped to maximize their aerodynamic efficiency to catch the wind — and even if you change their shape in one dimension — for example, the width of the blade — a bird flying at it from a different angle might only see its thickness instead of its width.

Laboratory tests have shown that birds do see some colors of blades better than others, but it depends on the background behind the blade. White blades show up very well against blue sky, so a bird approaching a turbine from the ground would see them better; but blue and green blades are more visible against typical ground colors. Black had the best average visibility across a variety of backgrounds.

Painting patterns, such as stripes, on blades didn't make much difference, in the same way that stripes on the blades of an electric fan just become motionless rings when the fan is turning fast enough. They were a little more visible when the stripes were at different places on different blades.

What turned out to work best was to paint one of the turbine blades — and only one — black. In laboratory tests, turbines with only one black blade seemed to be much more visible to birds. That's not surprising; for us, it would probably look like "blinking." But the real test is to find out in the field (literally) whether birds not only see a turbine with one black blade better, but also actually avoid colliding with it.

Between 2006 and 2016, a group of researchers tested this out on a wind farm in Norway, with four turbines chosen as "controls" that remained all white, and four nearby turbines as the experiments. For the first seven and a half years, all eight were completely white, and researchers collected data on bird strikes using dogs to search at regular intervals for bird carcasses on the ground around each turbine. Then they painted the four experimental turbines with one black blade and collected data for another three and a half years.

They found that the one black blade made a dramatic difference: Bird fatalities dropped by 70%. Raptor fatalities in particular were greatly reduced.

A lot more work needs to be done before victory can be declared. This is one wind farm in Norway; the experiment needs to be replicated in other parts of the world to see if all species of birds respond the same way. It's also challenging to try to interpret the data long-term, because if you reduce bird fatalities, you are changing the environment in which the experiment is happening: Presumably, the bird population will increase if there are fewer collision fatalities, and even if only a fixed percentage of birds die every year from collisions, if the population increases, then the absolute number of bird collisions would also increase. If you kept some number of turbines unchanged as a "control" for comparison, then you would be able to see whether the turbines with one black blade continued to make a difference, but once we repaint all the turbines in a wind farm with one black blade, it becomes more difficult to interpret the results because we no longer have anything to compare.

There is an urgency to reach a definitive answer to whether this solution works everywhere, since wind farms are being built out rapidly around the world. Painting one blade black is an easy thing to do when a turbine is being manufactured, but it's more difficult — and more expensive — to repaint a blade on a turbine that has already been installed. Assuming this turns out to be the best practice, the longer we wait to establish it as such, the more it will take to protect birds at existing wind farms.

But the good news is that after a long period of head-scratching, we have a good idea how to mitigate what is perhaps the most significant negative environmental impact of wind farms. Plus, it gave some researchers an excellent excuse to use a Rolling Stones song in the title of a stuffy research paper!

Paint it black: Efficacy of increased wind turbine rotor blade visibility to reduce avian fatalities

Featured image via barteverett/Shutterstock.com.