Wind turbines kill bats, and holding the blades still in low wind cuts deaths by 62%

Wind turbines kill bats, mostly 3 species

Wind turbines kill bats every year in the United States and Canada. Just 3 species make up most of the deaths, hoary bat, eastern red bat and silver haired bat, together 72% of every bat death recorded in a database the wind industry itself keeps. Sara Weaver, a wildlife ecologist at Bowman Consulting, says the reason is behavior, not chance.

Unlike birds, bats are actually investigating wind turbines and spending more time around them, which makes them at higher risk for mortality.

Sara Weaver, a wildlife ecologist at Bowman Consulting, quoted in source 5. Source 5.

Curtailment cuts bat deaths by 62%

The fix is called curtailment, holding the blades still until wind speed passes a set point called the cut in speed. Bats fly mostly in low wind, so raising the cut in speed keeps the blades still during the exact conditions when bats are near them. The newest and largest study of the practice, a 2024 statistical analysis of 29 tests across 6 wind facilities in North America over 10 years, found that raising the cut in speed to 5.0 meters per second cut bat deaths by 62% on average. The true reduction is probably between 54% and 69%, the study states, after adjusting for differences between sites and years. At that same 5.0 meters per second cut in speed, deaths fell 61% for eastern red bats, 52% for silver haired bats and 48% for hoary bats.

How to stop wind turbines killing bats

A separate review of 36 tests at 17 wind farms found that the size of the increase matters on its own. Raising the cut in speed by 0.5 to 1.3 meters per second cut deaths by 40%. A bigger rise, 1.5 to 2.3 meters per second, cut deaths by 59%. The largest rises tested, 2.6 meters per second or more, cut deaths by 63%. A wind farm in southwestern Victoria, Australia, tested a rise of just 1.5 meters per second after 6 fatalities of the southern bent winged bat, a species close to extinction, were recorded in its first 2 years of operation, and cut bat deaths there by 54%. Researchers used detection dogs to count carcasses before and after the change, and recorders to track bat activity nearby the whole time. Recorded activity did not drop during the test, so the fall in deaths cannot be explained by fewer bats being present, only by fewer of them being hit. Emma Bennett, the ecologist who led that study, says the size of the fix is out of proportion to the size of the change.

Any small increase in this cut in speed actually has a massive effect on bat survival.

Emma Bennett, wind and wildlife ecologist at Elmoby Ecology and lead author of source 3, quoted in source 5. Source 5.

A voluntary practice since 2015

United States wind operators adopted a voluntary best practice in 2015, feathering the blades, turning them to catch less wind so they slow or stop, below the cut in speed set by the manufacturer, during autumn migration, when the temperature is above 50 degrees Fahrenheit. Nearly a decade later, the practice is still not a required standard anywhere it applies. The studies above give a policy maker a number to weigh against that choice, a 62% average cut in a documented harm, from a change that costs only the power a turbine would have made in wind too light for bats to be at risk.