Golden eagles are not listed as threatened or endangered, but they are protected under other federal laws.
Two groups that sued unsuccessfully to block construction of the wind farm in 2011 say the government let the San Francisco-based company off last time with nothing more than a promise to do better at the site west of Great Basin National Park.
"There's no incentive for a wind energy developer to commit to anything if they are not held accountable for whacking birds they are not supposed to be whacking," said Rob Mrowka, senior scientist of the Center for Biological Diversity based in Tucson, Arizona.
"What they are basically saying is, 'OK, government, we are calling your bluff. We don't think you are going to do anything anyway,'" he told the AP.
Rene Braud, Pattern Energy's director of environmental compliance and policy, said possible golden eagle fatalities were anticipated when the Fish and Wildlife Service granted the necessary permit to open the wind farm in 2012. The facility has 66 giant turbines spread across about 12 square miles of land managed by the U.S. Bureau of Land Management.
"The project implemented mitigation measures, including power pole retrofits, to account for this," she said in an emailed statement. "It is unfortunate when any eagle is lost, and we will continue our efforts to reduce the project's impact."
The company submitted a request last year for another permit from the federal wildlife agency to allow for the incidental taking of birds protected under the Bald and Golden Eagle Protection Act. It has not yet been approved.
Pattern Energy believes other energy facilities and industries result in "far more bird fatalities than wind projects," Braud said.
"We would note that this is the second time over a period of more than two years of operations, while we have provided clean power to more than 45,000 homes each year with zero emissions," she said.
A lawyer for Western Watersheds Project, an Idaho-based environmental group that joined in the earlier legal effort to block the wind farm, said in a letter to the Fish and Wildlife Service and BLM earlier this week that it was "extremely disappointed" by the second fatality. The group demanded to know what would be done to prevent future deaths.
"The only mitigation done after the first golden eagle kill was a survey," staff attorney Kristin Ruether wrote. "We expect a far more robust response to this second unauthorized kill."