Why wind turbines kill so many bats comes down to 1 long summer peak, not 2

Why wind turbines kill so many bats

A study built specifically to test bat death timing pooled 10,291 bat carcasses with an exact discovery date, collected at 114 wind facilities across the United States from 2009 to 2021, and published in the journal PLOS ONE. The finding explains why wind turbines kill so many bats, a single, extended peak running from mid summer through early fall in every region studied, not the two separate seasonal spikes in spring and fall sometimes described. Only 1 species among those studied, the silver haired bat, shows any secondary rise in spring, and only in some regions. That two peak, spring and fall pattern actually describes a different group, long distance migrating woodland birds, while a third group, grassland and soaring birds that live inside wind energy areas year round rather than passing through twice a year, shows no clear seasonal pattern at all.

Bats investigate turbines, birds do not

Bats do not just fly past a turbine the way many birds do. They fly close to the blades and the tower and linger there, which puts them in danger far more often.

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 with Bowman Consulting, a Virginia based engineering firm that advises energy companies on cutting their environmental impact. Source 5.

That extra time spent near the spinning blades, not just flying through the area, is a reason timing through the year matters so much for how many bats die.

Which bat species die most at wind turbines

A separate peer reviewed count of 12,532 bat fatalities across 218 studies at 100 wind facilities found 82% of all recorded deaths happen from July through October, the same window the timing study identifies as the peak. The American Wind Wildlife Institute, a research group the wind industry itself supports, compiled a newer, larger database, 340 studies covering about 28% of installed United States wind capacity, and found 3 migratory tree bat species, the hoary bat, the eastern red bat and the silver haired bat, account for 72% of every recorded death. The hoary bat alone accounts for 30%, more than any other single species.

Fatality rates vary sharply by region

How many bats die at a turbine also depends heavily on where that turbine stands. The same American Wind Wildlife Institute database reports a median fatality rate of 8.4 bats per megawatt a year in the Midwest, the highest of any United States region studied, against 0.7 bats per megawatt a year in the Pacific region, the lowest, a median rate about 12 times higher in the Midwest than in the Pacific. The source itself cautions that part of this regional gap may come from differing monitoring rules between regions rather than differing bat risk alone.

Raising the turbine start speed cuts bat deaths

Knowing when the risk peaks is the basis for the 1 mitigation strategy shown to work. Operators can raise the wind speed at which turbine blades start spinning, a practice called curtailment, during the same low wind, late summer and fall nights when bats are most active. A meta analysis pooling 36 paired control and treatment studies at 17 sites in the United States and Canada found this cuts bat deaths by an average of 63%, and works best when the increase in wind speed is 2 meters a second or more. A 2 year curtailment study in Pennsylvania cut bat deaths by 44% to 93% after raising the turbine start speed from 3.5 meters a second to between 5 and 6.5 meters a second, according to reporting in Canary Media.