About an hour's drive east of San Francisco, steady winds blow about half the year through a stretch of terrain dotted with thousands of electricity-generating turbines.

Since the 1960s, Altamont Pass has been a proving ground for wind power. Now it's a test bed for solutions to one of the industry's biggest downsides: Turbines kill thousands of birds and bats annually.

The question of how to protect winged wildlife is becoming more pressing as deaths rise with the growth of wind power. Wind generation in the United States, for example, is up dramatically, but so are deaths of birds such as the federally protected golden eagle. (See related: "Federal Study Highlights Spike in Eagle Deaths at Wind Farms.")

Over the past several years, wind companies operating at Altamont Pass have made strides in reducing bird deaths across the wind resource area's 58 square miles (150 square kilometers). Part of the progress has come from shutting down in the winter months when the winds are low, and also from removing particularly hazardous turbines. (See related: "Notorious Altamont Wind Area Becomes Safer for Birds.")

Now a larger effort, overseen by the wind operators and the counties spanned by Altamont Pass, is under way to "repower" the area, or decommission old turbines and replace them with new ones that, ideally, will kill fewer birds. Alameda County is close to approving a one-year trial at Altamont of lower, "shrouded" turbines made by the Massachusetts-based company Ogin, according to county planning official Sandra Rivera. The new turbines have bonnet-like flaps that ring the perimeter of the turbine. The compact design is meant to boost efficiency, sit below a typical bird flight path, and deter a bird's flight into the rotors.

Trade-Offs With Newer Turbines

Thousands of older turbines at Altamont Pass have been fast-spinning and low to the ground, while also featuring cage-like lattice towers that were attractive places for birds to land and perch: a bad mix.

Most of these are being replaced by "monopole" towers, some as high as 500 feet (152 meters). The new towers are meant to be safer for wildlife, but a recent study suggests that may not be the case. Looking at hundreds of published reports on bird deaths at 59 wind farms across the United States, Oklahoma State University ecologist Scott Loss says the shift to the new, monopole designs is no simple fix.

The larger, more efficient structures appear to kill more birds per turbine than the windmills they're replacing—between three and eight birds per turbine per year, according to Loss. "Despite assertions that these turbines would reduce mortality rates, cumulatively they could still be responsible for a lot of mortality," said Loss, a co-author of the study. (See related story: "Sizing Up Wind Turbines: Bigger Means Greener, Study Says.")

One of the problems, Loss found, is that taller turbines kill more birds, particularly high-flying raptors and migrating waterbirds. So the benefits of the monopole design, and of having fewer, more efficient turbines, may be offset by the fact that new turbines are taller and tend to have larger rotor spans. At Altamont Pass, older lattice towers tend to range in height from 60 to 80 feet (18 to 24 meters). Newer models being installed are, in many cases, four to five times taller.

Loss estimates that between a quarter to half a million birds are killed by the new, larger turbines each year, a number that is sure to increase as wind power projects proliferate. Concern over bird deaths has derailed multiple high-profile wind energy projects recently, including a wind farm in the United Kingdom that conservationists claimed might threaten an endangered water bird.

Bird's-Eye View

In a bid to increase visibility to birds, turbines have been painted in high-contrast colors, fitted with noisemaking devices or lit with UV lamps to alert birds to spinning blades, but the efficacy of these experimental measures is so far unclear. In Spain, wind energy companies have worked with scientists to shut down wind farms when large numbers of migrating birds approach, a move that seems to work.

Researchers are beginning to look at the problem from a bird's-eye view, so to speak. Graham Martin, an ornithologist at the University of Birmingham in the UK, has conducted experiments with large migratory birds to understand how they see. It turns out birds don't pay much attention to what's ahead of them. They're focused instead on the ground or the area to their sides-either because they're hunting or looking for places to land. "People make easy assumptions that it's enough to stick something on a wire or paint blades," says Martin. "But the way we see these things is quite different than the way birds see them."

That may mean designing wind farms differently, perhaps by creating large areas on the ground that tempt the birds to descend to lower altitudes, out of the way of spinning turbine blades. "If you are trying to design something to alert birds to what's ahead, it's probably got to be a lot bigger and grander than you're thinking," Martin says. "You can't just make turbines more conspicuous."

That's a problem for the wind industry, which usually rents or leases space for windmills from farmers or other landowners but doesn't control the surrounding fields. "Less than two percent of the land area is permanently occupied by the infrastructure of a wind farm," said John Anderson, director of siting policy for the American Wind Energy Association. "Farmers are going to want additional compensation for measures like that." (See related story: "Planting Wind Energy on Farms May Help Crops, Say Researchers.")

There are other alternatives, of course: The U.S. Department of the Interior this month released a strategy to mitigate the impact of development projects, including wind farms, on birds and other wildlife. The plan focuses on putting windmills in places where they'll make the least impact, rather than trying to protect birds after the fact.

But the department also issued a rule last December that would grant 30-year permits to wind facilities allowing for limited numbers of eagles to be killed, granting them only to applicants "who commit to adaptive management measures to ensure the preservation of eagles."

The federal rule regarding these extended "take permits," as they are known, met with sharp criticism from conservationists. David Yarnold, head of the National Audubon Society, wrote in January that though "Audubon strongly supports properly-sited wind power," the Interior rule has "highly questionable conservation value," and called the department's assurances that there would be five-year inspections to police the rule "bureaucratic vapor." (See more about take permits and Audubon's take on eagles and wind farms here: "Wind Farm Faces Fine Over Golden Eagle's Death.")

Ultimately, though, no technology is without costs—and the advantages of renewable wind power over, say, coal-fired power plants may outweigh their impact on bird species. "Saying wind power can only be green if there are no impacts is like saying medicine can only be effective if it has no side effects," Anderson says. "At some point, we need to put the benefits and risks into context."

Oklahoma State's Loss, for example, notes that American homes and office buildings are responsible for hundreds of millions of dead birds per year, many times more than windmills. And researchers recently estimated that house cats kill well over a billion birds in the United States annually. "Comparing our numbers to total bird numbers, they might seem small, but that doesn't mean we shouldn't focus on local impacts on specific species, especially long-lived species like raptors or waterbirds," Loss says.

As wind energy proliferates in response to concerns over climate change, researchers will continue to work to mitigate its impacts. "We have to work around the birds—you can't just wag your finger and tell the birds to learn," said Martin. "There are no quick, easy fixes." (Take the related quiz: What You Don't Know About Wind Power.)

Additional reporting by Christina Nunez