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Bird Kills from Commercial Wind Farms: Fact or Fiction?


Dan Chiras, Ph.D.

The Evergreen Institute

Center for Renewable Energy and Green Building

While commercial wind machines do kill a small number birds, several scientific studies show that the problem has been exaggerated – grossly exaggerated. These studies indicate that bird kills from large commercial wind turbines pale in comparison to deaths from several common sources, among them domestic cats, electric transmission lines, windows, pesticides, motor vehicles, and communication towers (Table 1-1). Worldwide, hundreds of millions of birds – perhaps even billions -- are killed each year by these sources. Commercial wind turbines, on the other hand, kill a miniscule number of the birds. So why has wind gotten such a bad reputation?

Table 1-1

Estimated Annual Bird Dearth in the United States by Source

Activity/Source of Bird Mortality

Estimated Annual Mortality

Killed by cats

270 million or more

Collisions with and electrocution by electrical transmission wires

130 to 170 million

Collisions with windows

100 to 900 million

Poisoning by pesticides

67 million

Collisons with motor vehicles

60 million

Collisions with communications towers

40 to 50 million

Source: American Wind Energy Association

            Wind machines got a bad rap from one of America’s oldest and largest wind farms: the Altamont Pass Wind Resource area in California. Located just east of San Francisco, Altamont Pass is home to a mind-boggling 7,000 wind turbines. It is also the habitat of numerous raptors. Soon after the wind turbines were erected, the birds began to perch on the wind towers in search of abundant prey (ground squirrels and other rodents) that live year-round in the grasses at the base of the towers.

            Some scientists attributed bird deaths to a phenomenon called “target fixation.” Fixed on their prey, the raptors descended from their perches, often flying directly into the spinning blades. In addition to those that perished as a result of collision with the blades, studies suggest that 8 percent died from electrocution (from power lines), and 11 percent died when they collided with electric wires. The causes of death in the remaining 26% was unknown. Some researchers speculate that many raptors died from poisoning by rodenticides, pesticides used to poison rodents. These chemicals, the scientists hypothesized, may have moved up the food chain, poisoning the raptors.

            Although many raptors have died at Altamont, a two-year study of bird kill in the region revealed only 182 dead birds in two years. While any raptor death is of concern to those of us who cherish wildlife, the death rates at Altamont are insignificant compared to other factors.

Cats are probably the most lethal force that birds encounter. Scientists who have studied bird deaths from cats in Wichita, Kansas found that a single cat kills, on average, 4.2 birds per year. Other studies suggest even higher numbers. According to one study, a feral cat kills as many birds in one week as a large commercial wind turbine does in 1 to 2 years. Declawing a cat doesn’t seem to help much. According to one researcher, the majority of cats (83%) kill birds, even declawed and well-fed cats prey on wild birds. Neutering or spaying a cat does not seem to cut down on hunting, either. With more than 64 million cats in America alone, what’s the total loss?

No one knows for sure, but if the situation in Wisconsin is indicative of the national toll, America’s bird population is being decimated by our furry feline companions. In Wisconsin alone, researchers estimate that cats kill approximately 39 million birds per year. Nationwide, the number is estimated to be around 270 million, and is very likely much higher. Two scientists who studied the issue in one small town in England, estimated that cats kill 20 million birds per year in Great Britain alone. “Even if wind were used to generate 100% of U.S. electricity needs, at the current rate of bird kills, wind would account for only one of every 250 human-related bird deaths,” notes the AWEA. “So, if you want to save birds,” Mick jokes, “put your cat in a blender, then sign up for wind-generated ‘green electricity’ from your utility.” 

            Another 130 to 174 million birds die each year as a result of collisions with or electrocution by electrical transmission lines that crisscross the nation. Many victims are raptors, waterfowl, and other large birds. Birds are electrocuted when their wings bridge two hot wires.

            Another 100 million to 900 million birds perish after flying into windows, mostly in rural areas, according to another report.

            Pesticides kill an estimated 67 million birds each year. Scientists estimate that about 60 million birds die each year in the United States after being struck by motorized vehicles, according to the American Wind Energy Association’s report “Facts about Wind Energy and Birds.”

            Yet another 40 to 50 million birds perish after flying into communications towers and the guy wires that support them. Studies of one television transmitter tower in Eau Claire, Wisconsin, showed that it killed over 1,000 birds a night on 24 consecutive nights. This same tower killed a record 30,000 birds one evening!  A similar tower in Kansas killed 10,000 birds in a single evening.

            Another 1.25 million die as a result of collision with tall structures such as buildings, smoke stacks, and towers. 

Clearly, the Altamont Pass wind farm is benign compared to a host of other lethal factors. Altamont also appears to be an isolated case. No other wind farm in the United States experiences mortality rates remotely close to the Altamont Pass facility. Why? 

Acutely aware of the problem, contemporary wind developers select sites for new wind farms that are out of migratory pathways. Improvements in the design of commercial wind turbines have also helped to minimize bird kills at commercial wind farms. Over the years, wind machines have gotten taller, blades have gotten longer, and the speed at which the blades rotate has declined substantially. Large blades help harness more energy from the wind, but large, slow-moving blades are also more easily avoided by birds. Ever-larger commercial wind machines currently under development could reduce the risk even more.

“Overall, utility-scale wind is responsible for 0.01 to 0.02% of the total bird deaths in the United States,” according to Mick.  But what if we continuously increase the number of wind turbines in the U.S.?  


“Good point,” Mick responds.  “Double the number of turbines and we're up to 0.02 to 0.04%.  Increase the number by 1,000% and were up to 0.1%!  How many birds does habitat destruction such as mountain top removal to mine for coal kill--forever?!?”

To learn more about efforts to reduce bird deaths even more, checkout “Facts about Wind Energy and Birds” as the American Wind Energy Association’s web site,  Mick has written several articles on the topic, which you can check out at On the lower left, click on ‘Small Wind Toolboxes’.




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The Evergreen Institute provides high quality workshops in renewable energy and green building. Dan Chiras teaches practical, hands-on workshops education in solar and wind energy, energy efficiency, and green building.
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