Background Mortality of Wildlife on Renewable Energy Projects

With the expansion of utility-scale renewable energy development worldwide, accurate estimation of bird and bat fatalities is needed for informed policy-making and appropriate formulation of mitigation strategies. Background mortality, or the mortality caused by natural as opposed to anthropogenic processes, is often identified as a positive bias, and sometimes it is identified as a substantial or even leading contributor to fatality estimates. To estimate background mortality, I compiled fatalities/ha counted during searches of turbine-free study sites reported by others over 2548 ha and myself over 2297 ha. No bat fatalities were found in any of these searches. Bird fatalities/ha averaged 0.0055. I also compared estimates of fatalities/ha before and after turbine removals from 123 rows of wind turbines in California’s Altamont Pass Wind Resource Area (APWRA). These turbine rows had been searched for fatalities over various periods during 1998–2002 and 2006–2014, and fatalities had been recorded at each row during first searches of new monitoring periods. I used the same search methods as the monitor, but my first searches covered 624 ha of plots centered around vacant turbine sites. I found 0.0194 (95% CI: 0.0035–0.0352) bird fatalities/ha, but no bat fatalities. I estimated that background mortality was 3.6% (95% CI: 0–6.2%), mortality caused by unremoved power lines and meteorological towers was 8.2% (95% CI: 0–15.8%), and mortality caused by wind turbines was 88.2% (95% CI: 78–100%). Contamination of carcasses from operable wind turbines ≥ 400 m distant from vacant turbine sites likely biased my estimate upward by 3.5-fold compared to natural mortality averaged among sites far from wind turbines. This study does not support the notion that background mortality contributes substantially to mortality estimates at renewable energy projects.