Wind farms affect a variety of wildlife in different ways. However, bats stand out as being particularly impacted by wind turbines. Across most wind farms in Canada, just two or three bat species (Hoary Bat, Silver-haired Bat and Eastern Red Bat) account for most of the wildlife fatalities. At many sites in Alberta, Hoary Bats and Silver-haired Bats account for more fatalities than all songbirds combined. Some raptors (hawks, falcons and eagles) are also impacted by wind turbines, especially species such as Ferruginous Hawks that have limited ranges in Alberta and are already at risk from human activities.

Although many different bat species have been found dead at wind turbines, three species account for most fatalities in Canada: Silver-haired Bat, Hoary Bat, and Eastern Red Bat. These species share at least three important characteristics that may help explain their susceptibility: 1) they all undergo long-distance migrations during the spring and fall, with most likely spending the winter somewhere in the United States; 2) they all roost almost exclusively in trees during the summer; and 3) they are all well adapted to fly in the open, crossing long distances in short time periods.

In Alberta, Eastern Red Bats are not found dead in large numbers because the species is near the western limits of its geographic range, and is therefore relatively uncommon in the province (and nearly all individuals that do occur here are males). Eastern Red Bat fatalities increase towards eastern North America where the species is more common. All three species are long-distance migrants (most leave Canada for the winter) and most fatalities occur during fall migration. Resident species (such as our various Myotis species) are not found dead nearly as often at wind turbines, but this situation could change depending on turbine design and siting.

Bats are only killed by wind turbines while they are flying, which only occurs at night. Also, most deaths occur at low wind speeds—when turbines produce the least power. And most deaths occur while bats are migrating south during the late summer and fall. Not operating turbines during the night, during low wind speeds, and during fall migration can greatly reduce bat fatalities, with only a modest decrease in power generation.

Bats appear to be attracted to turbines. Several hypotheses have been suggested to explain this behaviour. Although the issue has not been fully resolved, the most likely explanation is that turbines are being mistaken for tall trees. Because the three bat species most often killed at turbines all roost and fly around trees, there is good reason for bats to seek out trees on the landscape (especially when crossing the open prairies, where trees are scarce). The tendency of migratory bats to fly towards turbines is problematic because it makes it difficult to predict risk from pre-construction surveys—bats won’t be there until the turbines are built! Not surprisingly, pre-construction bat surveys often perform poorly at predicting fatality rates at wind farms.

The wind energy industry is growing rapidly across North America and will soon become multiple times greater than what it was when experts first predicted precipitous declines in our migrating bat populations. This is a major concern for bats because 1) the rapid increase in the number of turbines does not leave adequate time to fully understand how populations will respond, and 2) any reductions in fatalities through the adoption of mitigation measures are quickly offset by increased fatalities from new turbines coming online.

Sources:

https://windexchange.energy.gov/maps-data/321
https://renewablesassociation.ca/by-the-numbers/

https://www.energy.gov/map-projected-growth-wind-industry-now-until-2050

There are proven methods for reducing bat fatalities, the most important being controlling when turbine blades spin. For example, turbine blades may be allowed to spin (“freewheel”) at low wind speeds, even when there is no electricity being generated. Bat fatalities could be reduced by up to 30% just by eliminating blade movement when wind is below the turbine’s cut-in speed. This results in no loss of electricity generation! Additionally, far fewer bats would be killed if wind energy companies were required to raise the turbine’s cut-in speeds (the wind speed at which rotors begin turning to generate electricity). For example, studies have shown that raising cut-in speed from 4.0 m/second to 5.5 m/second reduced fatalities by over half. Doing so does not heavily impact energy production (estimated 1-3% or less) because low wind speeds generate less power. Most energy companies are already planning on a much larger inherent range of uncertainty in wind conditions and thus energy production. Acoustic deterrents are being investigated as a potential way to reduce fatalities, but existing studies have reported mixed results, indicating that more research is needed before they are adopted for widespread use (see conservationevidence.com). However, even reducing fatalities by half may not be sufficient given the exponential growth in the number of turbines that is expected to continue over the next few decades. Unfortunately, despite there being proven methods to reduce bat fatalities, these measures remain largely voluntary and have not been required, so hundreds of thousands of bats have died each year that could potentially have been saved with only modest losses to electricity generation. More action is needed from state and provincial governments to compel adoption of proven ways to mitigate this threat.

Understanding the true scale of the problem that wind power development poses to bats requires us to consider what is happening across Alberta and North America as a whole. Each turbine in Alberta kills an average of 10.9 bats each year, and most of these fatalities have been Silver-haired Bats or Hoary Bats. There is evidence that the number of bats killed per turbine is decreasing over time, but a likely explanation for this trend is that migratory bat populations are declining (there are fewer bats remaining to be killed). There are now more than 900 turbines in Alberta and more than 77,000 across North America. In Alberta alone, annual fatalities are thought to be about 10,000 migratory bats per year, and many more of the bats that call Alberta home in the summer will die at turbines south of the border before they are able to return to Alberta the following year. Across North America, annual fatalities of migratory bats is likely in the hundreds of thousands of individuals. Bats are the longest-lived and slowest reproducing mammals for their size—their populations cannot withstand high mortality rates. This level of mortality is not believed to be sustainable and may result in the extinction of affected bat species if nothing is done.

Bats provide tremendous value to both the environment and the economy. The three species of migratory bats—Silver-haired Bat, Hoary Bat and Eastern Red Bat—are particularly adept at feeding on moths. Larval moths (cutworms, armyworms, and other caterpillars) can be major defoliators of crops and forests. These bats can eat their own body weight in insects per night, which helps make them one of the top predators of night-flying insects. Not only that, research has shown that just by being present in an area, bats create a ‘soundscape of fear‘ that helps reduce moth density, even when they are not directly consumed. It’s difficult to put a dollar value on bats, but one study estimated that bats could be worth between 3.7 to 53 billion dollars per year to the US agricultural sector alone.

But there is another great reason to protect bats—collectively, bats represent about a quarter of all mammal species worldwide.  If we fail to protect this important group, there could be an irreversible loss to biodiversity on this planet. The combined impact of white-nose syndrome and wind energy puts the future of bats in the country in great peril.