Abstract
With growing societal concerns regarding conventional energy sources like oil and natural gas, wind energy has become the dominate alternative energy source in North America and continued development is projected in both the U.S. and Canada. Wind turbines have contributed to bird and bat fatalities across North America. In certain locations, such as the forested northeast, bat fatalities have been especially high. As a result, conservationists are concerned about cumulative population-level impacts of bat fatalities at wind facilities should trends in fatality continue without mitigation. Studies have assessed operational mitigation, which raises the cut-in speed of turbines, at reducing bat fatalities only in relation to wind speed. Bat activity varies with environmental conditions like temperature, precipitation, and time of day. As such, researches have called for the need to investigate incorporating other weather variables into the design in order to fine-tune it. With current trends in bat fatalities and the exponential growth of wind installations across North America, immediate actions to reduce turbine-related bat fatalities is imperative to avoid potentially unrecoverable cumulative population-level impacts. In addition, further studies to investigate fatality estimates and patterns in regions across North America are needed to adequately assess impacts to bird and bat populations and inform and refine mitigation efforts. My objectives of this study were to: 1) estimate bird and bat fatalities at a wind facility in Vermont; 2) document patterns of birds and bats killed at turbines, including species composition, age, sex, and seasonal timing; 3) assess the relationship between bird and bat fatalities and environmental and site conditions; and 4) test the effectiveness of operational mitigation at reducing bat fatalities while incorporating temperature as a covariate to improve the design by fine-tuning it to weather conditions when bats are most active. I initiated a two year study from spring 2012 through fall 2013 at the Sheffield Wind Facility located in Sheffield, Vermont, U.S. From 1 April to 2 June and 1–31 October eight of the 16 turbines were searched daily and from 3 June to 30 September all 16 turbines were searched daily. Searchers walked along transects within designated study plots looking for carcasses. When a carcasses was recovered data on carcass location within study plot (e.g., distance from turbine, azimuth) and carcass characteristics (species, age, sex) were collected. I estimated fatalities using the U.S. Geological Survey’s Fatality Estimator software and adjusted for potential biases, including searcher efficiency, carcass persistence, and density-weight proportion areas. Bird fatalities occurred primarily during the spring and fall migration periods, with the majority of fatalities consisting of passerine species. Bird fatalities were estimated to be 211 (95% CI: 147, 321) for the site in 2012, with an estimated 5.27 per MW (95% CI: 3.68, 8.02), and 129 (95% CI: 60, 355) for the site in 2013, with an estimated 3.20 per MW (95% CI: 1.51, 8.86). Bat fatalities occurred primarily during the fall migration period and all bats recovered were migratory tree-roosting species, the majority of which were adult males. Bat fatalities were estimated to be 235 (95% CI: 160, 361) for the site in 2012, with an estimated 5.86 per MW (95% CI: 4.02, 9.02), and 46 (95% CI: 31, 71) for the site in 2013, with an estimated 1.12 per MW (95% CI: 0.76, 1.76). Bird fatalities were negatively influenced by wind direction and moon illumination, whereas bat fatalities were negatively influenced by wind speed and positively influenced by temperature. I found no difference in bird or bat fatalities among turbines with or without Federal Aviation Administration lighting or turbines located on two different mountain ridges. I also found no difference in bat fatalities at turbines with different rotor diameter sizes, but I did find significantly higher bird fatalities at turbines with a larger rotor diameter. I conducted the operational mitigation study from 3 June through 30 September of each year. I randomly selected 8 of the 16 turbines each night of the study for an equal number of nights at each turbine to cut-in at 6.0 m/s rather than the normal cut-in speed of 4.0 m/s. Treatments were implemented from half an hour before sunset to sunrise when wind speeds were <6.0 m/s and temperatures were >9.5° C. Bat mortalities at fully operational turbines were 2.7 times higher (95% CI: 1.9, 3.9) than mortalities at treatment turbines in 2012, resulting in an estimated 60% (95% CI: 29, 79) decrease in bat fatalities. In 2013, I found 1.5 times (95% CI: 0.38, 5.94) as many fatalities at fully operational turbines compared to treatment turbines. Few bats were found killed in 2013 and small sample sizes that year limited statistical power. Under the conditions of my study, incorporating temperature may not have reduced bat fatalities due to regional temperatures remaining above the 9.5°C threshold during peak bat fatality period. However, incorporating temperature did decrease the amount of unnecessary energy loss from implementing the treatment when bats are not active, particularly during late spring and early fall when temperatures normally drop below the 9.5°C threshold. As such, including temperature in the design can help improve the appeal of operational mitigation to wind companies by decreasing unnecessary costs. Energy loss from implementing the operational mitigation study was <3% for the study season and approximately 1% for the entire calendar year. Based on my findings, I recommend that operational mitigation be implemented during high risk periods to mitigate cumulative impacts to bat populations. I also recommend that future research continue to focus on fine-tuning operational mitigation by using other covariates that I did not test. Given the high number of bat fatalities at many wind facilities and the projected future development of wind installations across North America, it is essential for impact reduction strategies to be effective at reducing bat fatalities while also being cost efficient for broad implementation by the wind industry.