Why do birds get killed by wind turbines?

Lots of birds get killed by wind turbines—at least that is what you might think listening to the dire warnings about the environmental impact of wind turbines. The fact is that birds do get killed by wind turbines, but far fewer than you might think. As the American Wind Energy Association points out , many fewer birds are killed by wind turbines than by power lines, roads, or housecats. Unfortunately, that excuse is a red herring. It is like saying “Bobby stole a candybar” when the store clerk catches you stealing a piece of Hubba Bubba bubble gum.

The real question is: Are bird mortalities from wind farms an issue of concern from an environmental standpoint? The answer is perhaps. It depends upon how many birds are impacted, and which species. Two major components go into determining how many birds are killed–the first involves the location of the wind farm; the second is determined by bird behavior.

In this blog, I will be focus on the general reasons why birds get killed at wind turbines. I will also talk a little bit about how many birds are actually killed. Look for a discussion on the first question—wind turbine location—in a follow-up blog.

Driving a Bird to Work
So what makes birds run into turbines? I have a simple analogy to use when explaining bird kills to friends- just think about automobile accidents. When you get in your car to drive to work, you don’t expect to get into an accident. As a matter of fact, the odds are very low that you will have any kind of accident (about 1 in 100,000). When you multiply that by the million-or-so drivers on the road in a fair sized city, it becomes a virtual certainty that someone will have an accident in that city on any given day.

And, just like with you or me, there are circumstances that make accidents more likely. Imagine driving at night, in the fog or the rain; imagine driving in a new area, or even driving down an old familiar road, and suddenly there is a new construction zone. There are bird analogies for all of these events.

Birds can obviously see and avoid obstacles. When watching birds, you rarely see birds collide with objects. When driving, you can even watch them avoid your car with what sometimes appears to be a

Blackburnian Warbler during spring migration

startled expression. Some birds are good at this—you almost never see a dead swallow by the side of the road. Other birds are less good at avoiding obstacles. Sage Grouse are known for running into barb-wire fences. Let’s look at this in a little more detail using what is known about wind farms and bird behavior.

How many birds get killed?
Let’s start by looking at how many birds are actually killed at wind turbines. While birds do get killed, the numbers are in fact rather small. We don’t see thousands of birds dying at any turbine. We see an average of 2.8 birds killed per turbine† per year (calculated from data in NWCC 2010). For the statisticians out there, the plus or minus 2 standard errors is 2.1 to 3.5 birds/turbine/year.

Most of the birds killed are warblers, sparrows, and other small passerines (songbirds). Some are game birds such as pheasant. A few are raptors, waterfowl, or other species such as shorebirds, gulls, or egrets and cranes. The proportions of each type of birds killed are fairly constant across wind farms and regions (Morrison and Strickland 2008).

In 2003, with 4331 MW of installed wind power, bird mortalities were estimated to be about 9200 across the entire US (Erickson et al. 2005). This level of mortality is largely considered to be “insignificant” in the overall scheme. Most bird species can easily recover from a few hundred additional mortalities a year.

Mitigating factors—bird behavior
In fact, during migration when most birds are killed at wind farms, there are many millions of birds aloft. We don’t see most of them. Not only do we rarely look up, we are usually inside in our houses and cars shut off from the sky. Even when we do look up, we might not see the birds. Many of these birds migrate during at night when we are asleep or watching TV. Go outside and listen during the springtime and maybe you will hear the “tseep” of a warbler high overhead, or watch the full moon during fall and you will ever so occasionally see a bird fly across its surface, a silhouette against the moon.

Among the birds most heavily impacted at wind turbines are the night migrants, passerines such as the warblers and sparrows. Even these birds appear to avoid being struck by the turbines under most circumstances.

Many birds migrate so high you can’t see them. In fact, birds have been observed migrating as high as 30,000 feet up in the jet stream, and are occasionally a problem for airline traffic. These birds are so high that we usually don’t worry about them when siting wind farms (but see our comments under weather below).

Some birds, such as Whooping Cranes, fly along fairly narrow migration corridors and you won’t see them unless you are in the right place at the right time. In North Dakota, I once unexpectedly came upon two Whooping Cranes in an empty field—halfway between their wintering grounds in Aransas, TX and their breeding grounds in Canada. These giant but elegant birds danced a ballet—a bonding duet of spring.

So, just like humans, a few birds will have an “accident” while commuting. Birds occasionally run into buildings, light houses, and guy wires. I even had a bird run into me once. The harder an object is to see, the more likely they will run into it. The more an object looks like the background, or like a good path, the more likely they are to run into it. Windows and mirrored buildings are notorious for bird strikes (see BirdNotes 10 from Cornell Labs for more info on birds and windows).

Old-style wind turbines

Old-style wind turbines in California

Mostly, the night migrants fly well above turbine height. However, when it’s foggy or rainy, birds will fly much lower. This may cause a “double whammy” for the birds. Not only are they now flying within the rotor-swept height of the turbine, they may not be able to see as well and may have trouble orienting.

Even if birds can see the turbines, they may not avoid them. There is some evidence to suggest that birds don’t really pay much attention to visual cues during high-altitude migratory flights. Why should they? They evolved flying through relatively empty sky. In several million years there has never been a tree or a wall sticking up at 400 to 500 feet in the middle of the night sky. Imagine what you go through when you round a corner in your car and suddenly there is a tree in the road. Many of us take a second to register “Hey! That’s not supposed to be there!” before we apply our brakes.

Why do we care?
So, if so few birds are killed, why do we care? Why is it important to have biologists and regulators and non-profit organizations working so hard to “solve” this problem? You might ask, Is it really a problem?

For some, even if we can show that the impacts are low and unlikely to impact bird populations, it may still be a moral problem. People who care about life, who believe in the sanctity of all life, would prefer that no birds are killed at all. If some birds will inevitably be killed, they would prefer that every reasonable effort is made to minimize these deaths.

People who believe in the delicate balance of nature, or that humans do not fully understand the balance of nature, would also strongly argue for minimizing impacts. Our models and methods may be flawed, may not take into account subtle effects, or may not apply to this site at this time. While the large number of studies which show relatively consistent data are increasing biologists’ confidence, it is difficult to argue against this, as many technological advances in the past have had unanticipated impacts.

More turbines in California

Finally, while for many species biologists do not believe there will be significant impacts of any specific wind farm, there are potentially tens-of-thousands of wind farms (16 GW under one projection). The cumulative impacts—the impact of all these “small” effects added together—may have a large effect. The effect of all these wind turbines may add together incrementally or may have a “tipping point”; a point where suddenly the bird populations cannot overcome all of the insults; a point where the reproductive capacity is suddenly below the overall mortality levels. This point may vary by species or by group. In my next blog, I will talk about the different species and why impacts may matter more for some than for others, and which birds we may want to focus our conservation efforts.

(written by Caitlin Coberly 2011–Principle Ecologist at Merlin Ecological.  Please contact Caitlin at Merlinecological.com with questions or suggestions)

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