Due to incorrect information from NextEra Energy Resources, a front-page story Monday about new wind turbines in the Altamont Pass incorrectly described the size of their blades. They are 49 meters, or about 161 feet, long.

For years, environmentalists have raised alarms about the slaughter of red-tailed hawks, golden eagles and other raptors that have fallen victim to the whirling blades of thousands of wind turbines along the Altamont Pass in eastern Alameda and Contra Costa counties.

Now the most iconic wind farm in California is getting a major upgrade that promises to drastically reduce the number of bird deaths.

Nearly 2,000 of the 4,000 wind turbines in operation, many of which are nearly 30 years old, will be replaced over the next four years with about 100 huge state-of-the-art turbines that, at 430 feet, stand taller than the tallest coast redwood trees. For every new turbine installed, 23 of the old ones will be removed -- a dramatic drop expected to significantly reduce the number of birds killed each year.

The 50,000-acre Altamont Wind Resource Area, one of the nation's oldest wind farms, serves as a functioning museum of wind technology. Many of the older 100-foot-tall turbines are clustered closely together on ridges, each generating anywhere from 50 kilowatts to 750 kilowatts of power at peak production. Each of the new turbines, manufactured by Siemens, generates 2.3 megawatts of electricity -- enough to power 600 to 700 homes per year.


"It's almost a complete revolution in the way that you capture the wind," said Anthony Pedroni of NextEra Energy Resources, which owns about half of the wind turbines in the Altamont Wind Resource Area and is doing the upgrade. "The new turbines are 430 feet tall from the base to the tip of the blade, and the higher you go, the faster the wind speeds are."

The Altamont Pass was first developed for wind power in the late 1970s and at one point had nearly 6,000 turbines in operation. Much of the hilly land is owned by cattle ranchers who lease wind rights.

But the Altamont, which captures strong winds off the Pacific Ocean, is also a key migratory corridor and wintering spot for raptors. Studies have tried to quantify how many bats and birds are killed by turbines in the Altamont each year, but the task is difficult because scavengers often eat the corpses.

Shawn Smallwood, a renowned expert on birds and wind turbines, estimated that about 2,000 raptors are killed each year, along with as many as 8,000 other birds and bats. Young birds learning to fly are particularly vulnerable.

"Any way you look at the mortality data, there's been a tremendous impact on birds," said biologist Doug Bell, manager of the East Bay Regional Park District's wildlife program. "I've found birds sliced in half. You see all kinds of blunt force trauma."

'A video about the fatal attraction between birds and wind turbines from KQED's science series QUEST:

For more than a decade, wind companies and local chapters of the Audubon Society have struggled to find a compromise that maintains wind generation while reducing bird deaths. In December, Californians for Renewable Energy, five Bay Area chapters of the Audubon Society and the California Attorney General's Office reached an agreement with NextEra to "repower" the Altamont area with taller mega-turbines that some say could reduce the bird mortality rate by 80 percent.

NextEra has hired Smallwood as a consultant, and he is helping place the turbines to minimize bird deaths. Discussions with other wind developers in the area are ongoing.

"Birds and turbines are always an issue," said Michael Lynes, conservation director of the Golden Gate Audubon Society. "The best way to reduce avian mortality while keeping wind power in the Altamont is repowering with fewer turbines, and this is a step in the right direction. But we're always sober about it, because there will still be impacts to birds and bats."

The older turbines were built with a lattice structure that created many perching areas for raptors to hunt from, putting them in proximity to the blades. The new turbines are mounted on monopole towers that don't have perching areas, which should help keep birds safely away from the turbines.

"When they are diving and swooping down on their prey, they're not focused on turbines," Lynes said.

NextEra's repowering project will be done in three phases and is scheduled to be completed by 2015. The first phase is near Vasco Road and the Los Vaqueros Reservoir in Brentwood.

The sleek new turbines are majestic giants towering over the aging relics they are replacing. On a recent morning, a construction crew used a 315-foot-tall crane to lift a 180,000-pound unit that contains the gear box and generator to the top of the tower. Each of the three blades on a turbine is 150 feet long, nearly the width of a football field. In high winds, the tips of the blades spin at 180 mph.

Besides replacing turbines, NextEra will remove 6.5 miles of overhead electrical lines and about eight miles of road, allowing the land to return to a more natural state. The project also is creating union construction jobs: About 135 people have been hired.

Wind energy advocates say the collaboration between NextEra and environmental groups to reduce bird mortality marks an important milestone.

"Wind energy on a commercial scale was born in the Altamont," said Nancy Rader, executive director of the California Wind Energy Association. "Our understanding of its environmental impacts has evolved just as much as the technology has."


'Repowering' Altamont Pass

NextEra Energy is replacing 2,000 of the 4,000 wind turbines in operation at the Altamont Pass.
The old turbines are 100 feet tall and generate from 50 kilowatts to 750 kilowatts of power at peak production. The new turbines are 430 feet tall -- taller than coast redwoods -- and each generates 2.3 megawatts of electricity at peak, enough to power 600 to 700 homes a year.