Living with wind energy

Bats and turbines can share the sky.

Each summer, hoary and silver-haired bats raise their young in Alberta before leaving for the winter. These are the two species most often killed at wind farms in the province, with deaths peaking during fall migration. In its 2024 statement, the North American Society for Bat Research estimated that roughly 10,000 bats a year may be killed at Alberta's wind turbines, and possibly more than a million across North America in 2023. The Alberta figure was based on 900 turbines; the province now has about 1,800. In 2023, COSEWIC assessed both species, along with the eastern red bat, as Endangered, identifying wind turbine collisions as a substantial threat. Females raise one litter a year of one or two pups (up to four for eastern red bats), so turbine losses can add up faster than the population can replace them.

A 2021 study by Friedenberg and Frick modelled hoary bat populations across the United States and Canada as wind energy continues to expand. It found that turbine deaths may have already caused substantial declines. No one knows exactly how many hoary bats there are, because they roost alone in trees and migrate long distances; experts estimated between 1 and 10 million. Starting from their middle estimate of about 2.25 million in 2012, the model projected that the population would fall by half by 2028 without steps to reduce turbine deaths. That result held even when the model assumed the bats reproduced well and fewer new wind farms were built.

The encouraging part is that most of these losses are preventable. The majority happen on calm, low-wind nights during late summer and fall migration, exactly when turbines generate the least power. Slowing or feathering the blades until the wind picks up on those nights, a practice called curtailment, cut bat fatalities by about 60 percent in a southwestern Alberta trial. Across studies, the North American Society for Bat Research estimates average reductions of 68 to 80 percent for bats overall at a 6 m/s cut-in speed, the wind speed at which turbines start generating power, with blades feathered below that speed so they barely turn. The Society adds that larger reductions, which higher cut-in speeds can achieve, will be needed if bat populations are lower than experts expect, action is delayed, or curtailment isn't adopted everywhere. Friedenberg and Frick concluded that reducing bat deaths at wind farms quickly and widely could help manage the risk of hoary bat decline and extinction, though how far that mortality needs to be reduced is still uncertain.