BAT MORTALITY CAUSED BY WIND TURBINES. REVIEW OF IMPACTS AND MITIGATION MEASURES

Executive Summary

Most bat species in Québec are at risk owing, among other things, to the human-induced threats facing bat populations. Wind energy development is one of those threats. This document presents a review of the literature, with several objectives. First of all, we document the extent and impacts of bat collisions in wind farms in North America and Québec and we explain certain methodological biases that could affect mortality estimates. Then, we identify the factors that influence bat mortality in wind farms as well as the mitigation measures tested to date which have proven effective in reducing this mortality. Finally, we present a review of the application of the mitigation measures used in certain jurisdictions of North America.

According to the estimates in the literature, bat collisions with wind turbines total tens (even hundreds) of thousands of individuals a year in North America. However, it is difficult to compare wind energy projects among themselves, since the estimation of bat and bird mortality rates in wind farms is complex and evolving rapidly. These numbers are estimated based on carcass counts, corrected by an overall detection probability, which takes into account the area sampled, detection efficiency and carcass persistence. In 2016, we counted no less than three generations of estimators that have been applied to resolve this mathematical problem, with mixed success. Despite some uncertainty associated with mortality estimates, most authors agree that the main factor influencing bat activity, and therefore collisions with wind turbines, is wind speed. Bats are more active on nights with low wind speed (less than 6 m/s), and mortality rates are higher at these times.

  

Of the various mitigation measures studied, adjusting turbine cut-in speed is currently the only one that is clearly effective in reducing the number of bats killed while entailing relatively low implementation costs. Raising the cut-in threshold of wind turbines to 5 m/s reduced the number of bat mortalities by half, and raising the threshold to 6.5 m/s eliminated most collisions. Adjusting the cut-in speed caused financial losses equivalent to less than 1% of the annual production of wind power. Despite the scientific consensus on the effectiveness of this measure in reducing the number of bat collisions with wind turbines, it is not applied consistently. For example, Maine and Vermont have made it mandatory to increase the turbine cut-in speed in all their wind farms. Elsewhere in the United States, the members of the American Wind Energy Association (AWEA) voluntarily increase turbine cut-in speed during bats’ fall migration. Other jurisdictions, such as Ontario and Alberta, use a mortality threshold to initiate the shutdown of certain wind turbines.

In conclusion, the development of wind power poses a threat to bats, several species of which are at risk. For installed or operational wind turbines, mitigation measures such as raising the cut-in speed, shutdown or feathering during critical periods make it possible to significantly reduce bat mortality, while entailing relatively low implementation costs.

• Lemaître et al. 2017. Bat Mortality Caused by Wind Turbines: Review of Impacts and Mitigation Measures. Ministère des Forêts, de la Faune et des Parcs, Québec City, 26 p. [PDF]