Migratory bats, for some reason, have a lethal attraction to wind turbines. Now, they may get help via "feathering."

New industry guidelines, to be announced Thursday, aim to save tens of thousands of bats each year by idling turbines at low wind speeds during peak bat migration season. They could reduce by a third the number of bats killed at wind farms.

Seventeen members of the American Wind Energy Association, a trade group, have agreed voluntarily to begin idling, or feathering, turbines in the next year or two. Together, the companies produce nearly 90 percent of the wind power generated in the United States.

“It’s a big deal. That’s a big move on their part,” says U.S. Geological Survey bat biologist Paul Cryan. “It’s really encouraging to hear the industry is taking steps to curtail turbines, which is the best way we know of to reduce bat fatalities.”

Researchers and conservationists first raised the alarm about wind turbines killing bats more than a decade ago. Studies have since suggested that migratory bats, which roost in trees and fly long distances in the spring and fall, are attracted to the turbines and their towers for some reason. When they fly too close, they collide with the spinning turbine blades and are killed. Estimates of just how many bats are dying range from the tens of thousands to hundreds of thousands.

AWEA environmental director John Anderson says industry representatives have been discussing measures to reduce bat fatalities for more than a year, trying to find a balance between saving bats and generating power.

The discussions took on additional urgency when the northern long-eared bat, a cave-dwelling species that doesn’t migrate but is still sometimes found dead at wind energy sites, was listed as “threatened” by the U.S. Fish and Wildlife Service last year.

“That sent a signal that cave-dwelling bats were in trouble too, and the industry needed to step up and do its part,” says Tim Hayes, environmental director for Duke Energy Renewables, one of the companies that’s signed on to the agreement.

The “best practices” guidelines represent a fundamental change to how wind turbines are operated. Under normal circumstances, wind turbine blades are left to turn slowly in the wind until they reach “cut-in speed,” the point at which they’re spinning fast enough to begin generating power. For most turbines, that’s usually 3.5 meters per second, or about 7.5 miles per hour.

But even when they’re operating below the cut-in speed, wind turbines still spin — and take a toll on bats. The nocturnal animals don’t seem to be able to see the massive moving blades until it’s too late.

From now on, AWEA’s Anderson says, the industry will voluntarily program turbines to pivot parallel to the wind on calm nights, keeping their blades still. Wind turbines would be activated again when winds are strong to generate energy. “In terms of lost generation, there’s very little impact,” says Duke Energy’s Hayes.

The move should make a big difference for bats, though. Experiments at a site in Indiana in 2012 showed that feathering turbines when winds were slow reduced bat deaths by 30 percent on average. “Most bats were killed during low wind conditions, and we know bats are more active during low wind speeds,” says Bat Conservation International biologist Cris Hein. “This seems like low-hanging fruit to minimize bat fatalities.”

AWEA’s Anderson says that while power generation won’t be cut, the move will cost the industry millions of dollars, largely in additional wear and tear on windmills and in reprogramming older turbines. “The interest is to find the biggest conservation bang for the buck, with the least impact on generation,” says Anderson.

The new guidelines won’t be in effect year-round. They’re designed to cover bats’ fall migration period, typically mid-July through mid-October. “Eighty percent of fatalities occur during this three-month period,” says Hein, whose organization worked with the industry to research the most effective ways to avoid killing bats. “It’s the time when bats are migrating and mating.”

Researchers still aren’t sure what draws bats to wind turbines. But they do know bats are more likely to approach windmills on calm nights, a behavior that could be connected to how they land and roost in trees.

Feathering blades in the fall may be standard procedure nationwide by autumn 2016. Some companies are already well on their way to implementing the measures, and new installations are being designed with the bat-friendly guidelines in mind. “For new sites, we’re writing the turbine specifications so they operate that way right out of the box,” says Hayes.

There’s still a lot scientists don’t know, though. Migratory bats are notoriously hard to study, and researchers aren’t even sure where they go in the winter or how many of them there are. “It’s incredibly difficult to quantify the population levels of these species,” says Hein. “We don’t know what the impact is, or whether a 30 percent reduction is sustainable or not.”

But Cryan says the industry’s move could buy time for researchers to come up with even better ways to keep bats and wind turbines apart. Researchers are working on high-frequency noise generators that would be audible to bats and drive them away from wind turbines, ultraviolet lights to illuminate towers and repel bats, and behavioral studies that would help the industry fine-tune when it should feather turbines.

“The silver bullet would be some kind of device you could put on turbines to prevent bats and other creatures from approaching, and allow wind operators to go back to business as usual,” he says.

The story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.