At a time when demand is rising for greener energy sources, the Altamont Pass has become one of the nation's leading producers of wind power, generating enough pollution-free electricity annually to power 120,000 homes for a year.
But the Altamont, where more than 5,000 windmills line the hilltops, has also become a death trap for thousands of migrating birds that get chopped up in fast-rotating turbine blades as they fly through or hunt for prey.
An estimated 1,700 to 4,700 birds are killed each year in the 50-square-mile Altamont Pass Wind Resource Area, and of those fatalities, between 880 and 1,300 are federally protected raptors such as burrowing owls, red-tailed hawks and golden eagles, according to a study released last year by the California Energy Commission.
"Altamont is killing more birds of prey than any other wind farm in North America," said Jeff Miller, a wildlife advocate at the Center for Biological Diversity. "Incredible numbers of raptors are being killed there, and it's hard to believe it's not having effects on the populations."
Environmentalists were once reluctant to take on an industry that provides an eco-friendly alternative to fossil fuels blamed for air pollution and global warming. But the bird deaths have prompted wildlife advocates to sue nine wind farm operators and appeal Alameda County's decisions to renew their operating permits without requiring measures to reduce bird collisions.
"This industry has always wrapped itself in the mantle of green power and has sought to use the environmental benefits of wind power as an excuse for not doing anything about the environmental harms it causes," said Rick Wiebe, an attorney for the Center for Biological Diversity, one of the plaintiffs.
When some of the nation's first industrial windmills went up in the Altamont Pass more than two decades ago, few people thought seriously about the birds — even though the region is a major migratory corridor and hunting ground for raptors that prey on its abundant squirrels, gophers and rabbits.
Industry officials point out that turbines are only responsible for a tiny portion of human-caused bird deaths, compared with buildings, plate-glass windows, automobiles, pesticides and house cats.
"There are a bunch of other sources that are killing hundreds or thousands of times as many birds as wind turbines," said Tom Gray, AWEA's deputy executive director. "I don't want to minimize Altamont. It's definitely a legitimate problem what's happening with raptors there. But wind is not a threat to birds in general."
Still, windmill owners have taken steps to reduce bird deaths in the area. FPL Energy, which runs about half the Altamont's turbines, has already removed about 100 of its most deadly windmills and replaced another 169 with 31 larger, high-tech towers, said spokesman Steven Stengel.
Stengel warns, however, that requiring more extreme measures could put the wind farms out of business.
"There's a balancing act here," Stengel said. "We have to be able to reduce the bird mortality and at the same time allow the turbine operators to operate in an economically responsible manner."
Wind has become one of the fastest-growing sources of renewable energy, expanding about 20 percent annually over the past five years, according to the American Wind Energy Association. Last year, U.S. wind farms had the capacity to generate 6,740 megawatts of electricity. Another 2,500 megawatts is expected to be added this year.
While that's still less than 1 percent of the nation's electricity supply, the U.S. Energy Department wants wind power to comprise 5 percent by 2020.
Some environmentalists worry that Altamont's bird kill problem is hurting wind's environmentally friendly reputation.
"Allowing the taking of these protected species is giving a black eye to the wind power industry," said Michael Boyd, who heads Californians for Renewable Energy. "Wind is a good option, but you got to locate it appropriately and design it appropriately for the environment you're putting it in."