If you think wind energy is a good alternative to fossil fuels, but you also care about wildlife, you’ve probably worried about the possible "lawnmower" effect of spinning wind turbines on birds and bats.

At least some of that concern is justified. In the mid-1980s, people reported seeing piles of dead raptors at Altamont Pass Wind Resource Area near San Francisco, one of the nation’s first wind farms. When a Sierra Club employee later described wind turbines as the "Cuisinarts of the sky," newspapers went wild with reports of hashed-up hawks, and opponents of alternative energy seized on this new excuse to halt wind-farm development. Another alarm was set off in 2003, when it came to light that turbines at the Mountaineer Wind Energy Center in West Virginia had killed about 2,000 bats in a two-month period.

Wind turbines do kill birds and bats, but the scale of damage varies widely, depending on several factors, including the wind farm’s location, its turbine design, and the species of birds and bats that live nearby or migrate through. However, compared to the many other ways that humans kill winged animals, turbine blades generally cut only a sliver out of the pie.

According to the American Wind Energy Association, wind turbines account for only one out of every 5,000 to 10,000 human-caused bird kills nationwide. Many bird deaths are caused by communications towers, automobiles and domestic cats. The worst killers are glass windows: Researchers estimate that every year, 900 million birds die after slamming into these invisible barriers. But most of these victims are common city birds like pigeons and house sparrows.

Certain wind farms, like California’s Altamont, do pose a significant threat to raptors. Altamont, a cluster of wind projects begun in 1981, was built "in the absolute worst place to put a wind farm," says Jeff Miller, a spokesman for the Center for Biological Diversity. The Center is suing Altamont to force it to replace many of its older, less-efficient turbines with fewer and taller powerhouses; this would reduce the blade gantlet, Miller says, and at the same time increase energy production.

But Laurie Jodziewicz, communications and policy specialist for the American Wind Energy Association, says Altamont is unique. Not only is the wind farm located in the middle of a major migration route, it has 7,000 turbines that spin at the same elevation at which hunting raptors normally soar. Modern wind farmers have learned from Altamont, and now try to build outside bird migration routes, minimizing habitat destruction by choosing areas that have already been altered by industry. And the new cylindrical towers are harder for birds to nest on than the old ladder-like structures.

The U.S. currently generates only about one-half of 1 percent of its energy from wind, according to Jodziewicz. Comparing the new wind technology to other sources of bird mortality — such as the Exxon Valdez oil spill in the 1990s, which killed an estimated 500,000 birds — she says, "Even if the U.S. got all its energy from wind, the percentage of birds killed by turbines would be small."

Merlin Tuttle, executive director of Bat Conservation International, says scientists know almost nothing about the relationship between bats and wind energy, except that the wind farms reporting high numbers of dead bats are located close to forested areas, which are used by certain migratory bats. Some researchers speculate that the bats’ sonar may perceive the turbines’ rotating blades as flying insects. But so far, he says, wind energy companies are reluctant to give bat biologists the permits they need to research turbine-related bat kills.

Until scientists do more research, new wind farms may not know how to be bat-friendly. Even so, Ed Arnett, a conservation scientist with Bat Conservation International, believes that "wind energy is a great thing," and notes that better siting and turbine design may solve the bat-blade dilemma.