A long-awaited federal report on the environmental impact of wind power suggests birds have far more to fear from high buildings, power lines and cats than they do from the swirling blades of wind generators at Altamont Pass and elsewhere.

But North America's bats might have plenty more to worry about, according to a report released Thursday by the National Academy of Sciences.

The report said bats might be at considerable risk in the southwestern United States and elsewhere, where reliance on wind power has been growing. The wind-power turbines generate sounds and, possibly, electromagnetic fields that lure the acoustically sensitive creatures into the spinning blades, scientists suggested.

Until Thursday's report by the National Research Council, the research arm of the Academy of Sciences, most of the concern about the environmental effects of wind generators has focused on birds, but the report's statistics showed that wind turbines hardly make a dent in the bird population, which, it noted, faced greater danger from cats and other foes.

In the United States in 2003, wind generators accounted for only three-thousandths of 1 percent of bird killings -- no more than 37,000 birds. That same year, possibly as many as a billion birds died in collisions with buildings, and electrical power lines may have accounted for more than a billion more deaths, the report said. And domestic cats were responsible for the demise of an estimated hundreds of millions of songbirds and other species every year.

That aside, the report expressed concern about possible impacts from wind turbines on local bird populations, especially peregrine falcons and other raptors that are attracted to windy areas where the generators are likely to exist, and called for additional study. Raptors "are lower in abundance than many other bird species, have symbolic and emotional value to many Americans, and are protected by federal and state laws," the report noted.

The scientists' biggest concern was reserved for bats.

Recent analysis of bat kills amidst wind stations in the middle Atlantic states revealed more victims than were expected. The species impact could be significant partly because of an unrelated "decline in the populations of several species of bats in the eastern United States," the report said.

In the eastern United States, up to 41 bats are killed annually for every megawatt of wind energy generated along forested ridge tops, the report said. In Midwestern and Western states, the number is lower, no more than 9 dead bats per megawatt. Unfortunately, poor statistics about the size of bat populations -- which are notoriously more elusive than birds -- make it hard to estimate how severely such kills affect bat populations, the report said.

The panel of scientists, chaired by botanist-ecologist Professor Paul Risser of the University of Oklahoma, cited 11 hypotheses for why bats are attracted to wind generators. Among the more unusual possibilities is that the machines generate sounds, heat and electromagnetic fields that lure the creatures.

Or maybe curiosity killed the bat: "Because bats are curious animals, they may be killed as they explore novel objects in their environment," the report said.

On the cheerful side, the report noted that to date, no evidence exists that a single member of an endangered bat species has been kayoed by a wind generator. But, the scientists added, the statistics may not be complete, because when researchers sought dead bats around wind machines, bat corpses may have been missed or already absconded by predators.

The report calls for additional research into the problem, including the hypothesis that newer wind machines -- say, those with fewer but longer rotors -- will kill fewer birds and bats.