Our hunch is that the Altamont Pass California wind turbines, reportedly the site of some of the highest bird mortalities associated with any US wind farm, and using what is now an antique turbine design, are at the root of the widespread association of bird mortality with wind turbines in general. Now might be a good time to have a glance at this site, to get some perspective on the hundreds of raptors killed per year by the Altamont turbines.
If extrapolating the “worst case” rate is a bad idea, what about the “average” wind farm bird mortality figures? Even average rates, which are much lower or course, need to be looked at carefully.
To help our understanding of turbine hazards to birds we’d like to make an analogy, to your bicycle. Turn your bike upside down or put it in a work rack, set it to the highest gear…the one you use to go fast on a level slope…. and now move the wheel slowly with your hand. The chain moves rapidly with only a few degrees of wheel rotation. This symbolizes today’s cutting edge 1.5 mW turbines, which have a very large surface area of blade exposed to the wind and a gearbox that turns the dynamo quickly while the blades move slowly. Birds dodge these slow moving blades relatively easily.
Now put the bike in the lowest gear…the one you use to climb hills…and move the wheel with your hand fast enough to turn the chain as fast as before. That symbolizes the 20-year-old “bird-o-matic” wind turbine design. Small blades with small surface areas have to turn rapidly to overcome the magnetic force of the dynamos, which generate electricity.
Recapping: small blades, low surface area, lots of dead birds possible; very big blades, with large surface area exposed to wind, very few dead birds.
High capacity turbines are a relatively recent commercial product. Consequently, any field study of “avian mortality” done on a wind farm constructed prior to approximately the year 2000 (maybe a bit later in the US) is inappropriate for estimating bird mortality based on modern turbine designs.
Whether by intent or because older studies are more common, opponents of wind power will have cited bird mortality data from studies done before 2000 and, to make their point, are likely to focus on studies done on wind turbines erected in high exposure situations: e.g. in migratory pathways, at mountain passes, near nesting areas, and so on. Those are the numbers that get quoted at public hearings, published in the media, and that therefore underlie the collective consciousness about wind turbine hazard to birds. Not unlike what happens to people who constantly see fires crashes and shooting on the local news and come to think that what they are seeing is far more common than it really is, it all comes down to a risk communication problem.
Let’s frame the threat with a simple risk management equation: Mortality equals hazard times exposure, or M= H * E. Individual hazard (H) is the probability of Tweety being smashed to bits if it flies into a wind farm. The last four paragraphs helped establish that H is getting smaller, not bigger. This means average bird mortality is also getting smaller and will likely continue to do so. We remain optimistic that additional technological means will be discovered to further reduce “H” and therefore “M.” It might be as simple as avoiding any surfaces that would attract perching or nesting.
The exposure factor in the mortality equation (“E”) is a bit more complex. “E” is obviously highest where birds migrate, breed, and feed in flocks near wind farms. There are very windy places where “E” is low all year: a dearth of birds. And there are certainly windy places where “E” is high only during a brief migratory period, or for a limited number of species which fly at a certain elevation.
Certainly the siting process needs to steer wind farms away from places where it can be shown that “E” is relatively high. Designers continue to work on lowering “H,” while citizens, naturalists, municipalities with permitting or zoning authority, and scientists work to ensure that “E” is acceptably low. This is how it works. Once the turbines are up there’s no chance to alter “H” for at least another 20 years. “E” can change year to year, however, depending on something as basic as which crops are planted nearby. For this aspect mitigation planning can be a part of permit approval.
Statements about “average” bird mortality (“M”) do not well inform the debate over siting unless you get at the “H” and the “E” individually. By now it should be obvious that, like politics, all exposure is local. Citing an average “E” factor without some expert interpretation is not helpful. Having said that: here we go.
In the United States, cars and trucks wipe out millions of birds each year, while 100 million to 1 billion birds collide with windows. According to the 2001 National Wind Coordinating Committee study, “Avian Collisions with Wind Turbines: A Summary of Existing Studies and Comparisons to Other Sources of Avian Collision Mortality in the United States,” these non-wind mortalities compare with 2.19 bird deaths per turbine per year. That’s a long way from the sum mortality caused by the other sources.
For an excellent overview of all the major bird mortality categories we suggest you visit this site page maintained by the American Wind Energy Association
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