October 18, 2016 – Wind turbines are known to kill large birds that live nearby, but there is now evidence that birds from up to hundreds of miles away make up a significant portion of raptors killed at wind energy fields.

Using DNA from tissue and stable isotopes from the feathers of golden eagle carcasses, researchers from Purdue University and the U.S. Geological Survey found that golden eagles killed at the Altamont Pass Wind Resource Area (APWRA) in northern California can come from hundreds of miles away.

Understanding population-level differences and how individuals interact with turbines is key to meeting a U.S. Fish & Wildlife Service target of no net loss to their populations, explains Purdue.

The APWRA is one of the oldest wind farms in the United States, and one of the largest in the world originally with around 5000 turbines. Worldwide, such facilities have been responsible for the deaths of 140,000 to 328,000 birds and 500,000 to 1.6 million bats, says Purdue, raising questions about their effects on population sustainability.

“Eagles tend to use that habitat around the turbines. It’s windy there, so they can save energy and soar, and their preferred prey, California ground squirrels, is abundant there,” said J. Andrew DeWoody, a Purdue professor of genetics in the Department of Forestry & Natural Resources. “As they soar, these eagles are often looking straight down, and they fail to see the rapidly moving turbine blades. They get hit by the blades, and carcasses are found on the ground under the turbines.”

Collaborator David Nelson, an ecologist with the University of Maryland Center for Environmental Science, tested the birds’ feathers for stable hydrogen isotopes, which can be used to determine where the birds likely grew their feathers. The research team determined that about 75% of the 62 birds were from the local population. The remaining 25% likely migrated into the area before they were killed.

A genetic analysis revealed that golden eagles from the western U.S. have gene pools similar to those killed at APWRA, which reflects the capacity of these birds to disperse widely.

“The population models we built confirm that the age structure of the eagles killed at Altamont is difficult to replicate without substantial immigration,” said co-author Todd Katzner, a wildlife biologist with USGS. Katzner said these findings suggest environmental assessments of alternative energy facilities like Altamont Pass should consider not just local animal populations may be affected.

“If you only consider local birds in an environmental assessment, you’re not really evaluating the effect that facility may have on the entire population,” Katzner said.

The golden eagle population is a concern for several state and federal agencies, DeWoody added. He said future research could include looking at more bird species affected by turbines.

DeWoody said wind energy generators can receive permits that allow a certain number of unintended bird deaths but, if that number is too large, the companies could be fined. And knowing that a large percentage of the birds killed are from neighbouring states could compound the administrative issues.

The study was published in Conservation Biology, and funded by the U.S. Bureau of Land Management, U.S. Fish & Wildlife Service, and California Department of Fish & Wildlife.