The US government has, for the first time, enforced environmental laws protecting birds against wind farms, winning a million-dollar (£600,000) settlement from a power company.
The Obama administration has championed pollution-free wind power and used the same law against oil and power companies for drowning and electrocuting birds.
The case against Duke Energy and its renewable energy arm was the first prosecuted under the Migratory Bird Treaty Act against a wind energy company.
"In this plea agreement, Duke Energy Renewables acknowledges that it constructed these wind projects in a manner it knew beforehand would likely result in avian deaths," Robert Dreher, acting assistant attorney general for the US Justice Department's environment and natural resources division, said.
Dozens of eagles have died from wind turbines, including Duke's Top of the World farm outside Casper, the deadliest for eagles out of 15 similar US facilities operated by Duke Energy. The other wind farm included in the settlement is in nearby Campbell Hill.
"We deeply regret the impacts of golden eagles at two of our wind facilities," said Greg Wolf, president of Duke Energy Renewables. "Our goal is to provide the benefits of wind energy in the most environmentally responsible way possible."
A study by government biologists in September found that wind turbines had killed at least 67 bald and golden eagles since 2008. That did not include deaths at Altamont Pass, an area in northern California where wind farms kill an estimated 60 eagles a year.
Until now, no wind energy company had been prosecuted for a death of an eagle or other protected bird, even though each death breaks the law.
"Wind energy is not green if it is killing hundreds of thousands of birds," said George Fenwick, president of the American Bird Conservancy, which supports properly-sited wind farms. "The unfortunate reality is that the flagrant violations of the law seen in this case are widespread."
In 2009 Exxon Mobil pleaded guilty and paid 600,000 dollars (£370,370) for killing 85 birds in five states. Oil giant BP was fined 100 million dollars (£61.7m) for killing and harming migratory birds during the 2010 Gulf oil spill.
And PacifiCorp, which operates coal plants, paid more than 10.5 million dollars (£6.5m) in 2009 for electrocuting 232 eagles along power lines and at its sub-stations.
Wind farms are clusters of turbines as tall as 30-storey buildings, with spinning rotors as wide as a passenger jet's wingspan. Though the blades appear to move slowly, they can reach speeds up to 170mph at the tips, creating tornado-like vortexes.
Flying eagles do not look up as they scan for food and do not see the industrial turbine blades until it is too late.
The wind farms in yesterday's settlement came on line before the government drafted voluntary guidelines encouraging wind energy companies to work with the Fish and Wildlife Service to avoid locations that would affect wildlife.
Once a wind farm is built, there is little a company can do to stop the deaths. Some firms have tried using radar to detect birds and to shut down the turbines when they get too close. Others have used human spotters to warn when birds are flying too close to the blades. Another tactic has been to remove vegetation to reduce the prey the birds like to eat.
As part of the agreement, Duke Energy will continue to use field biologists to identify eagles and shut down turbines when they get too close. It will install new radar technology, similar to what is used in Afghanistan to track missiles, and will continue to voluntarily report all eagle and bird deaths to the government.
The Fish and Wildlife Service is investigating 18 other bird death cases involving wind turbines and about six have been referred to the Justice Department.