Post‐Construction Bat and Bird Fatality Study at the Blue Sky Green Field Wind Energy Center, Fond du Lac County, Wisconsin: July 21, 2008 – October 31, 2008 and March 15, 2009 – June 4, 2009


Title: Post‐Construction Bat and Bird Fatality Study at the Blue Sky Green Field Wind Energy Center, Fond du Lac County, Wisconsin: July 21, 2008 – October 31, 2008 and March 15, 2009 – June 4, 2009
Publication Date:
December 17, 2009
Pages: 62
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Gruver, J.; Sonnenburg, M.; Bay, K.; Erickson, W. (2009). Post‐Construction Bat and Bird Fatality Study at the Blue Sky Green Field Wind Energy Center, Fond du Lac County, Wisconsin: July 21, 2008 – October 31, 2008 and March 15, 2009 – June 4, 2009. Report by Western Ecosystems Technology Inc (WEST). pp 62.

The Blue Sky Green Field Wind Energy Center began commercial operation in May of 2008. The wind farm is located in the townships of Calumet and Marshfield in Fond du Lac County, Wisconsin. The wind farm consists of 88 Vestas V-80 wind turbines that rise to approximately 121 meters (397 feet) at the highest point at the top of a turbine blade. Each wind turbine is capable of generating 1.65 MW of electricity, and the wind farm as a whole is capable of generating 145 MW of electricity, or enough to provide power to 36,000 average residences. Here, we report results of fatality studies at Blue Sky Green Field that were conducted during the summer and fall of 2008 and spring of 2009. 


From July 21 to October 31, 2008, and again from March 17 to June 4, 2009, we conducted studies designed to estimate the number of bird and bat fatalities attributable to wind turbine operation. These dates correspond with the fall and spring migration periods for birds and bats. These studies included systematic searches at a random sample of 30 turbines at daily and weekly intervals. We also conducted trials designed to estimate potential sources of bias, including searcher efficiency and scavenger removal rates. We used a statistical estimator designed to accurately estimate total fatalities. The estimator started with the total number of fatalities found during searches and added amounts associated with the bias rates, search interval and proportion of area searched. We generated estimates of fatalities for all birds and all bats, as well as separate estimates for migratory and nonmigratory bat species.


For the entire study, we estimated 11.83 bird fatalities/turbine/year (9.08, 16.43; 90% C.I.) and 7.17 bird fatalities/MW/year (5.50, 9.94; 90% C.I.). We estimated 40.54 bat fatalities/turbine (30.98, 51.16; 90% C.I.) and 24.57 bat fatalities/MW (18.78, 31.03; 90% C.I.). This estimate includes 27 fatalities (12.2% of the total number used to generate fatality estimates) found on search plots outside of the scheduled searches and increased the overall estimate by at least 12% relative to excluding them. If the incidental finds are removed, the estimates would be 35.6 bat fatalities/turbine/year and 21.6 bat fatalities/MW/year. Fatality studies may not consistently include incidental finds of carcasses, or may not have any incidental fatalities to include, as the number of incidental finds likely depends heavily on total number of people active on site, number of inspections and/or visits to the turbine by other (non-study related) site personnel, number of visits to search plots for other activities, etc. 


Daily and weekly searches yielded estimates of 12.9 (8.81, 17.57; 90% C.I.) and 11.32 (7.72, 17.21; 90% C.I.) bird fatalities per turbine, respectively. We estimated 39.97 (30.76, 50.70; 90% C.I.) and 40.82 (28.16, 54.29; 90% C.I.) bats per turbine during daily and weekly, respectively. Non-migratory bat fatalities were estimated to be 23.37 (13.51, 27.78; 90% C.I.) and 20.23 (12.17, 26.14; 90% C.I.) per turbine for daily and weekly searches, respectively. Migratory bat fatalities were estimated to be 16.60 (12.99, 21.12; 90% C.I.) and 20.59 (16.33, 32.71; 90% C.I.) per turbine based on daily and weekly searches. The estimates for birds and bats are within the range of others from the United States and Canada but are higher than others reported from the Midwest to date. It may also be noted that estimates based on daily and weekly search intervals did not differ, which may be useful to consider with other goals when designing future studies.


We investigated the relationship between the number of fresh (< 1 day old) bat fatalities found during searches and local meteorological conditions measured on-site and at a local airport. The AIC-ranked best models indicated significant positive relationships between number of fresh fatalities and average temperature and low visibility conditions, and negative effects of wind speed at 50 meters and low cloud ceiling. The models also indicated a significant positive correlation between bat pass rates measured with ground-based detectors and number of fresh fatalities. 


This study represents one of the few currently available that we are aware of to estimate bird and bat fatalities from wind turbine operation in the Midwest and contributes to our understanding of wind energy impacts to birds and bats. As more wind power projects are built in the region, and additional studies become available, a clearer picture of the impacts to birds and bats will emerge.

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