The post-construction bird/bat mortality survey at the expanded Buffalo Mountain windfarm found an adjusted bat mortality rate of 63.9 bats/turbine/year. This figure is similar in magnitude to the bat mortality recorded in West Virginia (47.5 bats/turbine/year). Fewer bird strikes were recorded in this same survey.

Executive Summary

The Buffalo Mountain Windfarm (BMW) was constructed in Anderson County, Tennessee in late 2000, and consisted of three Vestas V47 wind turbines. The Tennessee Valley Authority's (TVA) environmental review process identified bird mortality as a likely environmental impact at the windfarm, and TVA conducted a three year study (September 2000-September 2003) to document bird mortality and activity at the windfarm; monitoring of bat activity and mortality was added after bat fatalities were also found. TVA documented a bird mortality rate of 7.3 birds/turbine/year and a bat mortality rate of 20.8 bats/turbine/year. Both of these mortality rates, which incorporate corrections for searcher efficiency, scavenging removal, and area searched, were high compared to other windfarms in the U.S.

In 2004, 15 additional turbines were constructed at BMW. These new turbines were larger than the three original turbines, and the magnitude of expected bird and bat mortality at the expanded windfarm was unknown. TVA conducted a second bird and bat mortality study during 2005, with the following objectives: 1) document bird and bat mortality at the original turbines and the new, larger turbines, and 2) compare results to the previous 2000-03 study.

Nine bird fatalities were recorded during searches at BMW in 2005, and the overall adjusted mortality rate was 1.8 birds/turbine/year. This rate is lower than those observed at BMW in previous years (7.3 birds/turbine/year), and similar to the national rate of 2.3 birds/turbine/year. All bird fatalities were songbirds, similar to fatalities at windfarms in West Virginia and Minnesota, but different from the numerous raptor fatalities observed at windfarms in the western U.S.

A total of 243 bats were found during searches in 2005, and the adjusted bat mortality rate at BMW was 63.9 bats/turbine/year, or an estimated 1,149 bats. This mortality rate is greater than the 2000-03 rate of 20.8 bats/turbine/year, considerably greater than the current national average of 3.4 bats/turbine/year, but similar in magnitude to a windfarm in West Virginia (47.5 bats/turbine/year).

Six species of bats were documented as fatalities at BMW during 2005. Eastern pipistrelle, red, and hoary bats were the most common species, making up 91% of the fatalities. None of the six species are listed as endangered or threatened. The majority of bat mortality (69%) occurred during late August and early September. This period coincides with the migratory season for bats. We also investigated correlations between mortality and environmental variables (wind speed and direction, temperature, pressure) and generation variables (generation, generation potential). None of these factors had significant associations with bat mortality.

Causes of bat mortality at windfarms are poorly understood. Turbine-specific factors (turbine lighting, height, and length of blades), likely relate to mortality. During 2005, no differences in bat mortality existed between turbines with and without lighting at BMW, but mortality rates differed between turbine types. Mortality rates were 35.2 and 69.6 bats/turbine/year at smaller and larger turbines, respectively. However, when mortality was measured per megawatt instead of per turbine, the smaller turbines had greater mortality (53.3 bats/megawatt/year) than the larger turbines (38.7 bats/megawatt/year).

Biotic factors, such as flight behavior of bats, their migration patterns, and aggregation of insect prey, may also contribute to bat mortality. An on-site radar study provided some insight into a few of these biotic factors, but more studies are needed to draw accurate conclusions.

Results of studies at BMW suggest that bird mortality at BMW is not a major cause for concern. However, the expansion of BMW has resulted in elevated levels of bat mortality that may be biologically significant for bats. Our findings mirror similar studies at other windfarms throughout the Appalachians, and suggest that a better understanding of bat mortality at wind turbines, and investigation of viable mitigation strategies, are needed in order to reduce levels of bat mortality.

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