If environmentalists and California state officials have their way, the towering windmills that dot the Altamont Pass will be replaced and moved to prevent the killing of thousands of birds annually, including species protected under federal and state laws.

In an effort to curtail the carnage, they say the turbines - which provide one-third of California's wind power - should be newer, taller models and be concentrated on the leeward side of the hills.

This spring, Alameda and Contra Costa counties, home to the state- designated Altamont Pass Wind Resource Area, will decide whether to include those recommendations and others when renewing permits on scattered parcels owned or leased by a dozen energy companies.

With 5,000 windmills in a 50-square-mile area, the Altamont Pass is the world's largest wind farm, producing enough electricity to power 200,000 households annually. But it is also the worst in the country for slaughtering birds.

Altamont Pass is a prime hunting ground for golden eagles and other raptors, and scientists estimate conservatively that the turbines kill some 4, 700 birds every year.

Unable to see the whirling blades, the birds fly into the turbines and get chopped up - nearly a third of them golden eagles, red-tailed hawks, American kestrels, burrowing owls and other raptors, according to a new study released by the California Energy Commission. About 70 golden eagles a year die.

"Bird kills are the single greatest negative environmental issue associated with wind power," said wildlife biologist Linda Spiegel, energy specialist at the commission, which is charged with resolving environmental problems from renewable energy sources. "Any time you put up wind turbines, you're going to kill birds. But there's nothing as bad as Altamont. Altamont happened before we knew there could be a problem."

Now, under pressure from the state and environmental groups, changes may be coming that would help protect the birds.

In 2003 and 2004, an eastern Alameda County zoning board renewed the permits for nearly all of the 4,000 turbines in the county, some into perpetuity at the request of the companies.

But after two appeals by the Center for Biological Diversity, Golden Gate Audubon Society and Californians for Renewable Energy, the contested permits will come up for a vote before the Alameda County Board of Supervisors in the spring. County Planning Department officials say they will revise the permits to include new recommendations from the state energy commission.

The study, conducted over a five-year span by scientists under contract to the agency, documented bird kills and recommended solutions such as replacing thousands of older turbines with fewer, larger-capacity modern turbines that - because of their greater height - don't appear to pose such a hazard to the birds. Scientists also advised clustering the windmills on the leeward sides of hills.

Clean wind energy is favored by environmental groups. But they join with the state energy commission officials in complaining that Altamont is giving wind power a bad name - and that it's time the industry invests in modern turbines.

"As it is now, Altamont is a red flag that opponents of wind power can wave in the industry's face," said Rick Wiebe, an attorney for the Center for Biological Diversity. "It's the combination that Altamont is one of the richest raptor habitats in the world with the highest density of breeding golden eagles in the world, together with the fact the companies are using this obsolete, first-generation killer technology.

"If you look at the 20 years that Altamont's been in existence, that works out to between 15,000 and 25,000 eagles, hawks and owls killed during that period of time. This can't go on."