WASHINGTON – Wind farms could generate as much as 7 percent of U.S. electricity in 15 years, but scientists want to spend more time studying the threat those spinning blades pose to birds and bats.

The towers appear most dangerous to night-migrating songbirds, bats and some hunting birds such as hawks and eagles. The risk is not well enough known to draw conclusions, a panel of the National Research Council said Thursday in a study requested by Congress.

“The human impacts of wind farms can be both positive and negative,” said Paul G. Risser, chairman of the committee that prepared the report.

Clearly the farms provide jobs and in some cases they can even be a recreational attraction, he said. But there also can be an effect on property values, and reflections off the rotor blades can be distracting to some people, said Risser of the University of Oklahoma and current acting director of the Smithsonian’s National Museum of Natural History.

Wind has powered sailing ships for thousands of years and has long been important to turn windmills that move water and grind grain. Only in recent years had the potential of the wind to generate electricity been tapped.

Wind farms generate electricity by using the wind to turn giant blades that rotate turbines to make power. The blades have diameters ranging from 230 to 295 feet and are mounted on towers 197 to 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 says estimates are that this source could generate 2 percent to 7 percent of the nation’s electricity within 15 years.

“There is a great diversity of opinion on how much there is going to be a ramping up of wind energy,” said report co-author Mary English of the University of Tennessee.

By reducing the need to generate electricity from burning fossil fuels, the turbines have been welcomed as a boon to the environment. Others worry about the danger to birds and bats, impacts on wildlife habitat and what some see as a blight on the scenery.

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

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.

. Wind generation in the mid-Atlantic highlands – elevated regions of Virginia, West Virginia, Maryland and Pennsylvania – is unlikely to reduce emissions of nitrous oxide and sulfur dioxide because current and future regulations will limit those emissions in Eastern states.

. In the mid-Atlantic highlands, preliminary studies indicate 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 in the Altamont Pass area. However, data are lacking for many facilities.

. While aesthetic concerns about proposed wind-energy projects often are the most heard, 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.