New research on bat roosts and wind power could help energy providers reduce collisions

Denver, Colorado — Mexican free-tailed bats tend to live in areas suitable for wind energy. A new study The findings, conducted by the U.S. Geological Survey and an international team of scientists, suggest that wind industry practices, such as shutting down wind turbines when wind speeds are low, may be more effective in reducing bat deaths than placing turbines in areas where bats are not present.

Wind energy is one of the fastest-growing sources of electricity generation in the United States, but wind turbines can be deadly to bats. The Mexican free-tailed bat is one of the bat species most commonly killed by wind turbines in the southwestern United States, accounting for more than 75% of the bat carcasses found in a survey of wind energy facilities in Texas.

The researchers found that of the more than 33,000 wind turbines currently in the southwestern United States, about a third are located within the summer and fall roosting areas of the Mexican free-tailed bat, primarily in western Texas, Oklahoma and Kansas. The bats’ roosting areas also overlap extensively with areas with high potential for future wind energy development.

“In the seven-state study region, the areas best suited for future wind energy are primarily in Texas, Oklahoma and Kansas, which are right in the middle of Mexican free-tailed bat roosts during the fall and spring,” Huang Dajian, assistant professor at Tamkang University in Taiwan and first author of this study.

The new study used Mexican free-tailed bat occurrence records and information on bat roost preferences to map their roosts for each season of the year, identifying areas that are best and least suitable for these bats. Then, for portions of their range within the United States, they mapped current locations of wind turbines and areas with high and low potential for future installation of wind energy. Putting these maps together, the researchers identified current and potential overlap of Mexican free-tailed bat roosts and wind energy to understand in which seasons and where Mexican free-tailed bats are most and least at risk.

While other studies have mapped bat roosting sites and wind energy, this is the first to consider the year-round cycle of these migratory bats. Mexican free-tailed bats winter primarily in central and southern Mexico. In early spring, most females and some males travel to the southwestern United States and northern Mexico, where they give birth and raise their young before returning to their wintering grounds in early fall.

These migration patterns influence when and where bats interact with wind energy. For example, the study found that during fall migration, bats are more dispersed, with nearly 10,000 wind turbines overlapping the most suitable migratory habitat, compared to spring migration, when just over 3,600 wind turbines overlap.

Wind energy developers can consider different strategies to limit impacts on bats and birds. One strategy is to select new wind energy sites that do not overlap with vulnerable species’ habitats. However, this may not always be possible: The new study sought to identify areas with high wind energy potential that do not overlap with the primary habitat of the Mexican free-tailed bat and found that few such sites exist year-round in the United States.

“Avoiding areas of potential impact is the first step to reducing the impact of any type of project. We want to find places where it’s a win-win, but throughout the annual life cycle, we haven’t found any of those places,” he said Jay Diffendorfer, research ecologist at the U.S. Geological Survey and author of the study“But studying the entire annual cycle gives us a deeper understanding of spatial and temporal dynamics — when species are sometimes present and sometimes not. This information can help wind energy companies make operational decisions.”

Win-win areas do exist where there is low overlap between wind turbines and bat roosts during the winter and spring, primarily in Texas, Oklahoma, and Kansas. In these cases, wind facilities can use bat deterrents or adjust wind turbine operations during the season when bats are present.

The open-access paper was published on July 24 in Nature Science ReportsThe research was conducted in 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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