There has been a lot of hubbub recently about wind farms killing birds and bats. Several media outlets have cited the “hypocrisy” of fining companies like Exxon for killing Eagles, but not penalizing wind farms for killing birds.
At Carbon Lighthouse we love both renewable energy and birds (though some of our team members’ love of pigeons is debatable), but we also love data and information-driven analysis. So we conducted a literature review of about 20 peer reviewed academic papers studying the effects of wind turbines. A selection of these papers is listed in the bibliography below. These papers not only carefully studied the effects of wind farms on birds and bats across the world, they studied those effects for years, sometimes even a decade or longer.
The results were clear: it turns out wind farms kill ten times fewer birds than fossil fuel power plants. Specifically, wind farms and nuclear power plants kill only 0.3 to 0.4 birds per GWh of electricity generated, while oil, gas, and coal power plants kill 5.4 birds per GWh of electricity generated. [2]
If we simply count up the birds based on the amount of electricity generated by each source, it means Wind Farms kill about 7,000 birds per year, Nuclear power plants kill about 327,000 birds per year, and coal and natural gas plants combined kill about 14,500,000 birds per year. [2]
These number are part of the reason why the National Audubon society “strongly supports” wind power.
We’ve compiled a simple table below showing concluding data from the studies, but before we launch in to that, let’s share some of the interesting anecdotes from some of these papers:
Interesting anecdote 1: A study of raptors and wind turbines found that the number of raptor deaths did not actually depend on the number of raptors near the wind turbines. In other words, the ten, er, oddest, raptors were going to be killed each year by wind turbines whether there were 100 raptors living near the wind farm, or 10,000. These are probably the same raptors that fly into the walls of your house.
Interesting anecdote 2: A study focused on Northumbria beach-based wind farms, a leading killer of birds as far as wind farms are concerned, found as many birds caught up in fishing gear or oil as by turbine blades. So stop leaving your pots of oil uncovered outside the house.
Interesting anecdote 3: Birds are killed much more often by fast moving turbine blades, which is reasonable since they are harder to see and harder to avoid. Bats, inexplicably, are killed in much greater numbers by the slower, easier to avoid blades.
Interesting anecdote 4: Your cat kills 2 birds per year on average. Your house kills 1.
Alright, on to the data! Each of the studies ([1], [3], [4], [5], [6], [7]) analyzed different sets of wind farms in different places. Some analyzed small wind farms, others large farms, others on shore vs. off shore. The following table is a compilation of the results from these studies:
|
Coastal (bird deaths per year) |
Land-Based (bird deaths per year) |
|
|
Small Turbine |
We don’t put small turbines on the coast |
6.7 to 11.2 if away from a breeding site, or 20 to 34.3 if the farm is located next to a breeding site. |
|
Medium Turbines |
We don’t put medium turbines on the coast |
1.19 |
|
Large Turbines |
16.5 – 21.5 bird deaths/year |
0.064 |
Larger turbine blades turn much more slowly so birds can easily avoid them. Thankfully, larger turbines are also more efficient and cost effective, so it is rare to see small turbines specified in wind farms any more.
Selected bibliography:
1. Newton & Little. Assessment of wind-farm and other bird casualties from carcasses found on a Northumbrian beach over an 11-year period. Bird Study, Vol. 56, July 2009.
2. Sovacool, Benjamin. Contextualizing avian mortality: A preliminary appraisal of bird and bat fatalities from wind, fossil-fuel, and nuclear electricity. Energy Policy, Vol. 37, June 2009.
3. de Lucas, Janss, Whitfield, & Ferrer. Collision fatality of raptors in wind farms does not depend on raptor abundance. Journal of Applied Ecology, Vol. 45, December 2008.
4. Smallwood & Thelander. Bird mortality in the Altamont Pass Wind Resource Area, California. Journal of Wildlife Management, Vol. 72, January 2008.
5. Kerlinger, Gehring, Erickson, Curry, Jain, & Guarnaccia. Night migrant fatalities and obstruction lighting at turbines in North America. Wilson Journal of Ornithology, Vol. 122, December 2010.
6. Everaert & Stienen. Impact of wind turbines on birds in Zeebrugge (Belgium). Biodiversity and Conservation, Vol. 16, November 2007.
7. Osborn, Higgins, Usgaard, Dieter, & Neiger. Bird mortality associated with wind turbines at the Buffalo Ridge wind resource area, Minnesota. American Midland Naturalist, Vol. 143, January 2000.
You mentioned the correlation of speed to turbine size, but I wonder if there are other effects. Bird strikes vs. electricity generated is probably the most appropriate metric to use, but one might look at this from several angles:
1. The larger tip diameter of large turbines means they sweep through a bigger area. It might be reasonable to expect that bird strikes per swept area would be constant. (But see below…)
2. Large turbines have lower rotational speeds, but the tip speed is (probably?) approximately the same for all size turbines. As you pointed out, we might expect the number of bird strikes to correlate with the (tangential) speed of the blade, which varies along the length of the blade. So there would be a direct, and probably non-linear, relationship between the turbine size and average tangential blade speed over its swept area — if you want to do the calculus.
3. Large turbines need higher towers to allow the blades to clear the ground (and also because they are robust enough to support the additional stresses), and are therefore catching higher velocity winds, leading to higher efficiencies. So unless the study normalizes for tower height, we can’t really compare apples to apples. The bird strikes per swept area per GWh produced should always be lower for large turbines — unless there is also an effect on the distribution of birds from ground level…
4. There may also be some improvement in both the airfoil and generator efficiency for large turbines, allowing them to convert more of that wind into electricity. So more apples to the small-turbine oranges, and less bird strikes for the large turbines.
Qualitatively, I would expect that the increased apparent efficiency of the larger turbines (due to tower height, airfoil, generator, etc) would dominate over the blade speed effect, and large turbines would have less bird strikes per GWh produced. Because of the apples-oranges issue, you could probably ‘normalize’ to any of the variables (e.g., bird strikes per swept area) and come to the same conclusion.
So it is obviously a complex correlation, and I didn’t answer any of my own questions. But I do have reason to wonder how I get anything done when I let my engineering mind out of its cage…
Nature is a very odd and interesting beast. While these papers and data do document the increased deaths of the birds due to wind power plants being there, perhaps a cost-benefit analysis would be useful here. Costs would be the potential loss quantified in a way which takes into account the reduced number of birds, but then again “survival of the fittest” is key. It would be interesting to see if any studies have been done to trace the evolution of the birds or the trend of the number of deaths per year after the construction of the windmills.