The towers appear most dangerous to night-migrating songbirds, bats and some hunting birds, but not enough about the risks is known to draw conclusions, the experts said in a study requested by Congress.

Panel chairman Paul Risser, a professor at the University of Oklahoma, urged states and local governments to use the report as a guideline for an "alignment of impacts" that they weigh when assessing whether and where to site wind farms.

Sources of carbon-free power, wind farms use the wind to turn giant blades that rotate turbines to make electricity. The blades have diameters ranging from 230 to 295 feet and are mounted on towers between 197 and 295 feet tall. Some farms contain hundreds of towers. The one at Altamont Pass, Calif., has more than 5,000.

Growing from almost nothing in 1980, wind powered turbines generated 11,605 megawatts of electricity in the United States in 2006, though that was still less than 1 percent of the national power supply.

Wind farms now operate in 36 states. The report, which the panel said was the most comprehensive look at the costs and benefits of wind power, says estimates are that wind could generate from 2 percent to 7 percent of the nation’s electricity within 15 years.

Many environmentalists and government officials welcome wind turbines given that they emit no pollutants. Others worry about the danger to birds and bats, impacts on wildlife habitat and what some see as a blight on the scenery.

Audubon backs wind, with caveat

Audubon, a national conservation group focusing on birds and other wildlife, has backed wind power as long as a farm is properly sited.

The new report "recognizes that properly sited wind power holds great promise as a source of renewable energy that can reduce global warming pollution," Audubon vice president Betsy Loyless said in a statement. "If we don"t find ways to reduce global warming pollution, far more birds and people will be threatened by climate change than by wind turbines."

"The first rule of avoiding negative impacts is a familiar adage: location, location, location," she added. "It is essential that industrywide environmental safeguards be developed so that each wind project can be considered on its own merits with appropriate studies before and after construction."

Overall, the report noted that the benefits of wind, such as reductions in air pollutants, benefit wide areas, while the environmental costs, such as effects on birds and bats, occur locally.

The panel cited estimates of turbine-related bird and bat deaths of between 20,000 and 37,000 in 2003. Some 9,000 of those were at the Altamont Pass site.

The Research Council, an arm of the National Academy of Sciences, concluded that:

By the year 2020 wind generators could offset as much as 4.5 percent of emissions of the greenhouse gas carbon dioxide from electricity production. The savings would be less in the mid-Atlantic states where there is less regular wind.

In the mid-Atlantic highlands, preliminary studies indicate that more bats are killed than expected based on experience with bats in other regions. There is not enough information to determine whether the number of bats killed will have overall effects on populations. However, there has been a regionwide decline in several species of bats in the eastern states, so the possibility of population effects is significant.

Turbines placed on ridges, as many are in the mid-Atlantic highlands, appear to have a higher probability of causing bat fatalities than those at many other sites.

At current levels of use, there is no evidence that fatalities caused by wind turbines result in measurable demographic changes to bird populations nationwide, with the possible exception of raptor fatalities at Altamont Pass, a facility with older, smaller turbines that appear more apt to kill such birds than newer turbines are. However, data are lacking for  many facilities.

While aesthetic concerns often are the most heard about proposed wind-energy projects, few decision processes adequately address them.

Other potential human impacts include effects on cultural resources such as historic, sacred, archaeological and recreation sites and the potential for electromagnetic interference with television and radio broadcasting, cellular phones and radar.

Building wind farms requires clearing land and soil disruption and has the potential for erosion and noise.

Regulation of wind farms is a developing area and better technical guidance to the costs and benefits needs to be made available. This guidance could be developed by state and local governments working with groups composed of wind-energy developers and non-governmental organizations representing all views of wind energy, the committee said.

The National Academy is an independent organization chartered by Congress to advise the government on scientific matters.

The report did not examine the impact of offshore wind-energy projects.

PIN/MSNBC