Reporting from Oakland — Scores of protected golden eagles have been dying each year after colliding with the blades of about 5,000 wind turbines along the ridgelines of the Bay Area's Altamont Pass Wind Resource Area, raising troubling questions about the state's push for alternative power sources.
The death count, averaging 67 a year for three decades, worries field biologists because the turbines, which have been providing thousands of homes with emissions-free electricity since the 1980s, lie within a region of rolling grasslands and riparian canyons containing one of the highest densities of nesting golden eagles in the United States.
Video: Golden eagles at Altamont Pass
"It would take 167 pairs of local nesting golden eagles to produce enough young to compensate for their mortality rate related to wind energy production," said field biologist Doug Bell, manager of East Bay Regional Park District's wildlife program. "We only have 60 pairs."
The fate of the Bay Area's golden eagles highlights the complex issues facing wildlife authorities, wind turbine companies and regulatory agencies as they promote renewable energy development in the Altamont Pass and across the nation and adds urgency to efforts to make the technology safer for wildlife, including bats, thousands of which are killed each year by wind turbines.
Gov. Jerry Brown in April signed into law a mandate that a third of the electricity used in California come from renewable sources, including wind and solar, by 2020. The new law is the most aggressive of any state.
The development and delivery of renewable energy is also one of the highest priorities of the Interior Department, which recently proposed voluntary guidelines for the sighting and operation of wind farms. Environmental organizations led by the American Bird Conservancy had called for mandatory standards.
The U.S. Fish and Wildlife Service authorizes limited incidental mortality and disturbance of eagles at wind facilities, provided the operators take measures to mitigate the losses by replacing older turbines with newer models that are meant to be less hazardous to birds, removing turbines located in the paths of hunting raptors and turning off certain turbines during periods of heavy bird migration. So far, no wind energy company has been prosecuted by federal wildlife authorities in connection with the death of birds protected by the Migratory Bird Act, the Bald and Golden Eagle Protection Act or the federal Endangered Species Act.
The survival of the Bay Area's golden eagles may depend on data gathered by trapping and banding and then monitoring their behavior in the wilds and in wind farms.
On a recent weekday, Bell shinnied up the gnarled branches of an old oak to a bathtub-sized golden eagle nest overlooking a canyon about 25 miles south of Altamont Pass.
Two fluffy white and black chicks, blinked and hissed nervously as he scooped them up and placed them into cloth sacks. He attached the sacks to a rope and delicately lowered them 45 feet to the ground.
"As adults, these birds could eventually wind up anywhere in the Western United States or Mexico — that is, if they live that long," Bell said.
With field biologist Joe DiDonato, he banded the birds' legs and recorded their vital statistics in a journal that chronicles more than a decade of raptor research in the region. The message is a grim one.
Each year, about 2,000 raptors are killed in the Altamont Pass by wind turbines, according to on-site surveys conducted by field biologists. The toll, however, could be higher because bird carcasses are quickly removed by scavengers.
Environmentalists have persuaded the energy industry and federal authorities — often through litigation — to modify the size, shape and placement of wind turbines. Last year, five local Audubon chapters, the California attorney general's office and Californians for Renewable Energy reached an agreement with NextEra Energy Resources to expedite the replacement of its old wind turbines in the Altamont Pass with new, taller models less likely to harm birds such as golden eagles and burrowing owls that tend to fly low.
The neighboring Buena Vista Wind Energy Project recently replaced 179 aging wind turbines with 38 newer and more powerful 1-megawatt turbines. That repowering effort has reduced fatality rates by 79% for all raptor species and 50% for golden eagles, according to a study by Shawn Smallwood, an expert on raptor ecology in wind farms.
It remains unclear, however, whether such mega-turbines would produce similar results elsewhere, or reduce fatalities among bats.
Nationwide, about 440,000 birds are killed at wind farms each year, according to the Fish and Wildlife Service. The American Wind Energy Assn., an industry lobbying group, points out that far more birds are killed each year by collisions with radio towers, tall buildings, airplanes, vehicles and in encounters with hungry household cats.