Minnesota regulators last week rejected a wind farm developer's plan for protecting eagles, bats and other wildlife from its turbines as inadequate.
"Inadequate" is also a good way to describe our knowledge of how wind farms actually affect eagle populations.
"There's just not a lot of good scientific studies that have looked long term at this whole interaction," says Dr. Julia Ponder, director of the Raptor Center, a renowned large bird rehabilitation center in St. Paul.
The number of reported eagle-turbine collisions is not large, says Ponder. She's aware of just five cases ever involving bald eagles. An average of 67 golden eagles are killed annually at a single cluster of wind farms in California's Altamont Pass, but only around 50 golden eagles have been killed anywhere else since the industry began, she says.
To put those numbers in some context, the Raptor Center sees more than 100 eagles per year, more than any other facility of its type. Of the 122 eagles brought to its facility last year, 32 were brought in for lead poisoning (only three survived). The majority were there for some type of unknown trauma, the specific cause of which is usually impossible to pinpoint.
The types of injuries the Raptor Center sees suggest that hunting (lead poisoning), farming (pesticide poisoning), and driving (hit by cars) pose greater risks to eagles than wind turbines.
Comparing these risks is difficult, though, because there is no central database of eagle mortality causes. The Raptor Centers' own numbers are biased because they only see birds that people find and bring in, skewing the sample towards those that are injured in cities or along roadsides.
It's unclear whether the small number of reported eagle-turbine collisions is actually reflective of the true number, says Ponder. And it's unknown why collisions numbers are higher for golden eagles than bald eagles, though Ponder suspects it has to do with the geographic location of existing wind farms overlapping more with golden eagle habitat than bald eagle territory.
How many eagle-turbine collisions occur might not even be the right question to ask, says Ponder. She points to a 2010 study of white-tailed eagles near a 68-turbine wind farm on the island of Smøla in western Norway, which showed the number of nesting pairs within 500 meters of the wind farm fell from 13 before construction to only five pairs a few years later.
"It appears that some of them just said, 'OK, we don't want to breed here anymore,'" says Ponder.
As a scientist, though, Ponder says what's most concerning to her is that developers, policymakers and others are making decisions about the issue without the benefit of research.
"At this point I would say that the data is not being collected," says Ponder, "and therefore the decisions are being made, as they often are, for reasons other than scientific validity."