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DENVER, Colo. — Mexican free-tailed bats tend to live in the same areas that are good for wind energy, according to a new study by the U.S. Geological Survey and an international team of scientists. These findings suggest that wind industry practices like turning off wind turbines during low wind speeds may be a more effective strategy for reducing bat fatalities than placing turbines in areas where these bats are not found. 

Wind energy is one of the fastest-growing sources of electricity generation in the U.S., but wind turbines can be deadly for bats. Mexican free-tailed bats are among the bat species most frequently killed by wind turbines in the southwestern United States, making up more than 75 percent of bat carcasses found in surveys at wind energy facilities in Texas. 

The researchers found that about a third of the more than 33,000 currently existing wind turbines within the southwestern US are in highly suitable summer and fall habitat for Mexican free-tailed bats, primarily in western Texas, Oklahoma and Kansas. The bat’s habitat also had broad overlap with areas that have high potential for future wind energy development. 

“Across the seven-state study area, the areas most suitable for future wind energy were mainly in Texas, Oklahoma and Kansas, and a lot of these areas happen to be right smack in the middle of where Mexican free-tailed bats are in the fall and spring,” said Ta-Ken Huang, Assistant Professor at Tamkang University in Taiwan and lead author on the study. 

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The new study used records of Mexican free-tailed bat occurrence and information about the bat’s habitat preferences to map their habitats in each season of the year, identifying regions most and least suited for these bats. Then, for the part of their range located in the United States, they mapped the current locations of wind turbines as well as areas with high and low potential for future wind energy installation. Putting the maps together, the researchers identified the current and potential overlap of Mexican free-tailed bat habitat and wind energy to understand where and in which seasons Mexican free-tailed bats are most and least at risk. 

While other studies have mapped bat habitats and wind energy, this was the first study to consider the full annual cycle for these migratory bats. Mexican free-tailed bats spend the winter primarily in central and southern Mexico. By early spring, most females and some males depart for the southwestern US and northern Mexico, where they give birth and rear their young before returning to their wintering grounds in early fall. 

This migratory pattern influences where and when the bats may interact with wind energy. For example, the study found that during the fall migration, the bats are more spread out and nearly 10,000 wind turbines overlap with the most suitable migratory habitat, increasing the risk compared to the spring migration, when just over 3,600 turbines overlap. 

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Wind energy developers can consider different strategies to limit impacts to bats and birds. One strategy is to select sites for new wind energy that do not overlap with the habitats of vulnerable species. However, that may not always be possible: the new study sought to identify regions that had high potential for wind energy but did not overlap with prime Mexican free-tailed bat habitat and found that few such places existed year-round in the United States. 

“Avoiding areas where impacts may occur is the very first step used to reduce impacts from any type of project. We were hopeful that we would find win-win places, and across the whole annual life cycle, we didn’t find that,” said Jay Diffendorfer, USGS Research Ecologist and author on the study. “But studying the whole annual cycle gives us a more refined understanding of the space and time dynamics—the species is present some of the time and not there other times. That information can help a wind energy company make operational decisions.” 

Win-win areas, with low overlap between wind turbines and bat habitat, do exist in the winter and spring, primarily in Texas, Oklahoma and Kansas. In this situation, wind energy facilities could use bat deterrents or adjust wind turbine operations during just those times of year when bats are present. 

The open-access paper was published on July 24 in Nature Scientific Reports. The research was a collaboration with Tamkang University in Taiwan, the University of Arizona, Florida State University, the University of Tennessee, and the National Autonomous University of Mexico. 

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