Long before wind turbines sprouted on Altamont Pass, it was home to the highest density of golden eagles in the world and their major breeding area in the United States.

Almost as soon as the first turbine started rotating, the bird carcasses started piling up: Golden eagles, burrowing owls, red-tailed hawks, other raptors, western meadowlarks and migrating songbirds.

Raptors searching for prey such as ground squirrels and gophers will glide over the ground at low height, unaware of the rapidly moving tips of the turbine blades. They also perching on the turbines themselves - a lesser danger, as the birds are much more able to recognize the danger near the axis where there is almost continual motion.

Eagles are protected by the Bald and Golden Eagle Protection Act and the Migratory Bird Treaty Act of 1918, both of which prohibit the killing of these birds. Burrowing owls also are listed as a species of special concern in California.

Altamont Pass provides a productive area for wind-generated electrical power. As warm air rises in the Central Valley, it draws in cold air from the ocean across the pronounced gap in the Coastal Range. It also yielded an average near $60 million of yearly revenue during the first five years of this century.

In 2003 and 2004, the operating companies proposed doubling the permitted generating power by adding 2,600 turbines. By this point, the annual number of bird deaths, estimated from carcass counts, were: Golden eagles 100, burrowing owls 380; red-tailed hawks, 300; and American kestrels, 333.

The prestigious Predatory Bird Research Group (PBRG) at the University of California, Santa Cruz, conducted a three-year study (1994-1997) of golden eagle mortality from wind turbines and concluded, "the population of golden eagles ... remains intact."

This surprising result is understandable by splitting the eagle population into three groups: Juveniles, breeding pairs and "floaters," breeding age eagles that have not found a nest and are available to replace a lost mate. It appears that these floaters make up the majority of the dead eagles.

While everyone appreciates power generation without the emission of carbon dioxide, the Golden Gate Audubon Society, the Center for Biological Diversity (CBD) and an allied group sued to demand an environmental review of the expanded permits. In a settlement they agreed, with the exception of the CBD, to a delay until November 2009.

During the ensuing period, the operators agreed to perform some inexpensive mitigation measures, including reducing turbine operation during the breeding period of January and February, painting the rotors and minor measures which it was hoped would reduce raptor mortality by half.

On Feb. 12, an interim report on raptor mortality during 2005-2007 was released.

Instead of a reduction in raptor mortality, the study found deaths had risen except for that among golden eagles, which had fallen to the sustainable level of 49 deaths per year.

Burrowing owl mortality suffered the greatest increase - more than 300 percent - and the overall raptor deaths almost doubled.

Although this report casts doubt on the efficacy of the settlement and the stipulated mitigation measures, the Audubon Group, CBD, the turbine owners and operators and the California Energy Commission will all anxiously await the final results in November 2009.