Making the Sky Safer for Bats

hen BCI Research Scientist CJ Campbell, Ph.D., first set out to study bat migration during her graduate program, she thought she knew what she’d find. After all, you don’t have to be a scientist to understand the basics of typical seasonal migration: as temperatures drop, people and animals alike tend to head toward warmer climates.

But a paper that Campbell and colleagues recently published suggests that winter migration is far more complicated for certain bat species. They found that two of the three bat species most affected by wind turbine collisions—the hoary bat (Lasiurus cinereus) and Eastern red bat (Lasiurus borealis)—don’t follow this typical pattern.

Together, the hoary bat, Eastern red bat, and silver-haired bat (Lasionycteris noctivagans) represent about 75% of fatal bat collisions with wind turbines in the U.S. Hoary bats make up about one-third of these collisions and are of particular concern. In 2017, BCI Chief Scientist Winifred Frick, Ph.D., published a paper with colleagues demonstrating that current wind energy fatality rates could reduce the population of the hoary bat by as much as 90% over 50 years.

Most fatalities occur during late July and August, during the autumn migratory period. However, little is known about bat movement patterns during that time of year.

“That is really important, because we want more green energy, and we want it to be wildlife-friendly,” Campbell says.

Campbell and her colleagues knew that having a better understanding of these species’ migration patterns could help find solutions that reduce deadly collisions with wind turbines, so they set out to study their migratory patterns.

Conducting that research was more difficult than it sounds. “Bat migration, particularly over long distances, is incredibly hard to study,” Campbell explains. Bats are challenging to catch, and even if you do catch them, they’re too small for most GPS transmitters that might be deployed for tracking larger animals. “In most circumstances, we know almost nothing about long-distance bat migration, so we have to get very creative in the methods we use to study it,” Campbell says.

To better understand the migration patterns of the three main species of interest, Campbell and her team analyzed chemical signatures in the bats’ fur through a process called stable isotope analysis.

This technique uses predictable variations in the weight of hydrogen found in rainwater to tell scientists where a raindrop fell. “When looking at the isotopes, the rainwater in Florida is going to look very different from the water in New England,” Campbell says. As bats drink and forage in their local environments, the “isotopic signature” of local rainwater is incorporated into their fur.

Throughout their migratory journeys, “you are what you eat, in a very literal way,” says Campbell. By analyzing the fur of thousands of individual bats, she and her team forensically reconstructed the previous movements of those individuals.

The team expected to find “to-and-fro migration,” which Campbell says is when bats “spend the summer somewhere in northern areas, then fly south in the fall and overwinter somewhere balmier, and then fly back north in the spring.” She describes this migratory pattern as “the default we expect to see in most seasonal migrations,” like with most songbirds.

However, she found that hoary bats and Eastern red bats weren’t following this typical pattern. Instead, their migration pattern was far more complex and unexpected. When Campbell was analyzing the results, she repeatedly reviewed the data and results, but the signal remained atypical from what she would have expected from seasonal bird migrations. These bats were flying in long, wandering routes in the autumn, sometimes traveling hundreds of kilometers northward from their summer homes before eventually traveling back southward to warmer areas to overwinter.

The team theorizes that this “pell-mell” migration strategy may help the bats pursue mating or feeding opportunities. “There are probably good reasons for the bats to do this, even if we don’t yet understand what they are,” Campbell says.

Michael Whitby, BCI’s Director of Bats and Wind Energy Research, says understanding bat migration is useful for fine-tuning the practice of curtailment, which involves turning wind turbines off during the periods of highest risk, such as when bats are known to be migrating through an area. This is the most effective strategy for reducing bat deaths from collisions with turbines.

The more we know about where and when bats are migrating, the more precisely wind turbine curtailment strategies can be deployed to avoid bat fatalities and energy loss.

“It’s always challenging to work in market-driven sectors,” Whitby says. “Conservation is one element in a complex decision-making environment for industry, so making changes can be slow.”

However, when research can demonstrate best practices with implementable solutions, it becomes easier to influence policy and advocate for wind facilities to adopt those practices.

Over the last year, BCI has worked on multiple fronts to raise awareness about the importance of expanding wind energy infrastructure in a way that also protects bat populations. Through the Bat Mitigation Working Group, a task force of the United Nations Convention on the Conservation of Migratory Species, BCI recently published two fact sheets on migratory bats and the bats and wind energy issue to inform policymakers about the importance of bats and the need to implement mitigation solutions that protect vulnerable species.

Campbell emphasizes that expanding renewable energy capacity through wind energy development could have a positive impact on bat conservation since reducing climate change caused by fossil fuels benefits all living things, including bats. But it’s crucial to ensure that wind facilities are constructed and operated to minimize their impacts on wildlife.

“Bats are at risk everywhere,” Campbell says. “We are really concerned about the magnitude of bat fatalities and are urgently looking for solutions to better place and operate facilities to be as friendly to bats as possible.”