USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
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Research Topics Wildlife & Fish

Mitigating Impacts of Wind Energy Development on Populations of Migratory Bats

windfarm at dusk

Energy development and distribution promises to be a major global challenge during the 21st century.

In the US, there are increasing calls to achieve greater energy independence and there is general agreement that renewable sources of energy will be an important component of our energy portfolio.

Because wind energy is able to generate electricity without many of the environmental impacts associated with other energy sources, it is expected to produce a net benefit to wildlife species. However, in some situations, wind energy developments have produced large impacts to bats in the form of direct mortality from collision with moving turbines.

The majority of bats killed, worldwide, are migratory species, which in California are represented largely by hoary bats, red bats, silver-haired bats, and Mexican free-tailed bats. A number of hypotheses exist for why bats collide with wind turbines (Kunz et al. 2007 Cryan and Barclay 2009), but from a practical standpoint there are two important avenues of exploration that may increase compatibility between wildlife and wind energy.

First, predictive tools must be developed that allow us to ascertain whether proposed wind energy developments are likely to be a threat to bats. Second, once facilities are built, we must take steps to minimize impacts to bat populations.

Hoary Bat (Laisurus cinereus) Cibola National Forest, New Mexico
Hoary Bat (Laisurus cinereus) Cibola National Forest, New Mexico. (Gabe Reyes)

Our work on this subject which focuses both on predicting risks and designing effective and efficient mitigation strategies, was a natural outgrowth of our work to develop quantitative methods to assess bat populations. Echolocation monitoring during the pre-construction period is the primary tool used to assess risk to bat populations from proposed wind energy developments. However it remains unclear whether this method is an accurate predictor of fatalities during facility operations.

What's more, the number, type, and height of detectors required to achieve an accurate characterization of bat activity in an area has not been quantified. We combine the use of continuously operational echolocation detectors with the use site-occupancy estimation techniques to simultaneously characterize bat activity levels and quantify the number of detectors necessary to precisely estimate those activity levels (Weller and Baldwin, 2011).

weather monitoring equipment
Echolcation monitoring equipment mounted on meteorological tower at Iberdrola Renewables' Manzana project, Kern County, California.

Further, by linking echolocation activity of bats to meteorological conditions measured on-site we develop models to predict bat activity based on day of the year and the prevailing weather conditions.

Research has shown that increasing the wind speed at which turbines begin to generate energy can decrease the number of bat fatalities with marginal changes in annual power production (Arnett et al. 2011). Our models provide a tool for improving the efficiency and effectiveness of this mitigation by specifying nights when probability of bat occupancy are reduced even during low wind speeds.

To date, our field work has been conducted in the dry southeast portion of California, but the analytical tools and techniques are broadly applicable to wind energy facilities worldwide. This work has been funded largely by the California Energy Commission's Public Interest Energy Research (PIER) environmental research program and is done in collaboration with the Bat and Wind Energy Cooperative and Iberdrola Renewables.

Literature cited:
  • Arnett, E. B., Huso, M.M., Schirmacher, M.R., Hayes, J.P. 2011. Altering turbine speed reduces bat mortality at wind-energy facilities. Frontiers in Ecology and the Environment 9:209-214.
  • Baerwald, E.F., Edworthy, J., Holder, M., Barclay, R.M.R. 2009. A large-scale mitigation experiment to reduce bat fatalities at wind energy facilities. Journal of Wildlife Management 73:1077-1081.
  • Cryan, P.M., Barclay, R.M.R. 2009. Causes of bat fatalities at wind turbines: hypotheses and predictions. Journal of Mammalogy 90:1330-1340.
  • Kunz, T.H., Arnett, E.B., Erickson, W.P., Hoar, A.R., Johnson, G.D., Larkin, R.P., Strickland, M.D., Thresher, R.W., Tuttle, M.D. 2007. Ecological impacts of wind energy development on bats: questions, research needs, and hypotheses. Frontiers in Ecology and the Environment 5:315-324.
  • Weller, Theodore J.; Baldwin, James A. 2011. Using echolocation monitoring to model bat occupancy and inform mitigations at wind energy facilities. Journal of Wildlife Management. In Press.