Mincing machines: an estimated 40,000 birds die a year in US wind turbine blades — conservationists worry that rare raptor populations are at risk. Credit: G. LEAPER/ECOSCENE/CORBIS

What's 3% of a bird? The last seven centimetres of a swan's wingspan? The right foot of an ostrich? Or the annual death toll attributable to an average wind turbine? In the context of last week's report1 by the US National Academy of Sciences (NAS) on the environmental impacts of wind-energy projects, it's the third definition that counts. It takes 30-odd turbines to reach a kill-rate of one bird a year.

The scientists who wrote the report naturally attached lots of caveats to this figure, which they gleaned from 14 studies they felt were of good quality. They acknowledged that rates can differ widely from site to site, and that although, as Hamlet said, there is a special providence in the fall of a sparrow, such a fall might not be quite as special, or worth avoiding, as the death of a bald eagle.

In the final analysis, though, whichever way you slice it, or them, America's birds seem to die in turbine blades at a rate no higher than 40,000 a year. Deaths due to domestic cats, on the other hand, are put at “hundreds of millions”. It is possible, the panel noted, that the turbines are rather worse for bats; recent studies have turned up more of their carcasses than expected. But the numbers are still small.

The shadow of the waxwing slain

It is unlikely, though, that the study will allay the worries of bird-lovers who look on wind farms with loathing. For carbon-free power sources, wind turbines have an oddly bad reputation among conservationists: bird safety, like landscape aesthetics, is a common cause for complaint.

And the wind farms do not have a completely clean bill of health. As the NAS report pointed out, much of the data available is too narrow and site specific. “My personal opinion is that the evidence base is very poor,” agrees Andrew Pullin, head of the Birmingham, UK-based Centre for Evidence-Based Conservation. The Royal Society for the Protection of Birds (RSPB), a British charity with a large membership and quite a lot of muscle, points to the fact that while its members oppose large offshore developments, evidence on British wind farms is limited to studies of small installations onshore. It has also taken a vociferous stance against a vast 234-turbine wind farm planned for the Isle of Lewis in the Scottish Hebrides.

In Spain, the world's number three wind-power producer after the United States and Germany, published studies also suggest that the number of birds killed is low. But Spanish environmentalists feel the figures aren't telling the full story. Alvaro Camiña, an environmental consultant who monitors bird fatalities at 70 of the country's 140 wind-power farms, says that in the case of a widely accepted study published in 2004 (ref. 2), the field work was completed a decade earlier when turbines were much smaller.

Camiña, who is paid by the regional governments of Rioja, Valencia and Andalucia, recently submitted a report on his research to the Ministry of Environment in Madrid. It is due to be released soon. Of particular importance, he says, are the number of raptors killed — for example, 866 griffon vultures (Gyps fulvus) since 2000. “It's important to know the mortality of large birds because they have a lower number of offspring. Even a small number of deaths can affect a population.”

I heard that more than 1,000 birds a year run into the Washington monument. Should we tear that down?

Raptors have long been a cause célèbre in the United States as well. The wind farms in California's Altamont pass have been cutting down golden eagles (Aquila chrysaetos) since they were opened in the 1980s. But Rick Koebbe, president of PowerWorks, a California firm that owns turbines in Altamont, argues that this should be put into context.

“I heard that over 1,000 birds a year run into the Washington Monument. Should we tear that down? We're out here trying to do a job to save the Earth. We even save birds, since they are twice as vulnerable to pollution as humans.”

Unsurprisingly enough, Koebe is against any further regulation of his industry: “If you give the Fish and Wildlife Service control over the wind-power industry,” he says moodily, “there will be no more wind power.”