Bat activity rates do not predict bat fatality rates at wind energy facilities

Abstract

Bats are found as fatalities at most wind energy facilities around the world, creating a challenge for wind developers to predict risk to bats in an area before building a new facility. Bat echolocation activity surveys are the standard method for assessing risk, but their effectiveness has not been demonstrated. Sites with relatively low pre-construction bat activity rates are predicted to yield relatively low post-construction fatality rates (i.e., low risk), and vice-versa. To test this hypothesis, we ran simple linear regressions on bat activity rates and fatality rates from 49 paired pre- and post-construction studies across the United States and Canada. Bat activity rates did not predict bat fatality rates at wind energy facilities by detector height, by call frequency category of bats, or by season (P > 0.10). One possible explanation for the lack of a predictive relationship is that bat activity patterns may change between the pre- and post-construction periods if bats are attracted to turbines. Indeed, we found support that bat activity rates increased across call frequency category and season at four wind facilities that had measured bat activity rates before (× = 1.89 bat passes/detector-night) and after turbines were built (× = 4.84 bat passes/detector-night). However, simple linear regressions of post-construction activity rates and fatality rates from 25 studies found no correlation between activity and fatality rates collected concurrently by detector height (ground, raised, nacelle) or by call frequency category (P > 0.05). We conclude that the current pre-construction survey methods of collecting bat activity rates at proposed wind energy facilities do not provide reliable information on how many bat fatalities there may be once the facility is built, and advocate exploring other methods for assessing risk to bats prior to wind development.