ReviewEstimates of bird collision mortality at wind facilities in the contiguous United States
Introduction
Wind energy has emerged globally as a promising alternative to fossil fuels. As of June 2013, more than 270 gigawatts (GW) of power generation capacity were installed across the world’s >13,000 wind facilities (The Wind Power, 2013). Roughly 20% of this capacity is installed in the United States (American Wind Energy Association, 2013), providing enough energy to power 18 million households. A continued increase of U.S. wind energy development is expected in response to the Department of Energy’s (DOE) goal to have 20% of total energy generated from wind power by 2030 (U.S. DOE, 2008). Conservationists have expressed concern about direct and indirect impacts of wind energy development on wildlife, including bird and bat collisions with wind turbines (Kunz et al., 2007a, Kunz et al., 2007b, Kuvlesky et al., 2007), habitat loss, and creation of barriers to wildlife movement (Drewitt and Langston, 2006, Kuvlesky et al., 2007, Pruett et al., 2009, Kiesecker et al., 2011). Despite the decommissioning of many lattice-tower turbines that have caused large numbers of bird collisions, such as those at Altamont Pass in California (California Energy Commission, 1989, Smallwood and Karas, 2009), bird collisions still occur at turbines with solid monopole towers (e.g. Johnson et al., 2002, Kerns and Kerlinger, 2004), which now comprise the vast majority of U.S. turbines.
Wildlife mortality from collisions with wind turbines is the most direct, visible, and well-documented impact of wind energy development. However, conclusions about collision rates and impacts of collisions on bird populations are tentative because most of the mortality data is in industry reports that are not subjected to scientific peer review or available to the public (Piorkowski et al., 2012). The accessible data—which could provide a non-representative sample of all studies—suggests that bird collision rates at turbines are lower than at other structures, such as communication towers, buildings, and power lines (Drewitt and Langston, 2006), and that mass collision events are rare at wind facilities (but see Johnson et al., 2002, Kerns and Kerlinger, 2004, American Bird Conservancy, 2011). Pre-construction assessment of collision risk at proposed wind facilities has been unreliable, with no clear link documented between predicted risk levels and post-construction mortality rates, likely due to substantial variation in collision rates among turbines and a failure to consider risks at individual proposed turbine sites (de Lucas et al., 2012a, de Lucas et al., 2012b, Ferrer et al., 2012). In addition, most risk assessments focus on the total numbers of birds predicted to be present at a site. A failure to consider species-specific risks may result in relatively high post-construction rates of mortality for some species even if total bird mortality is relatively low (Ferrer et al., 2012).
Mean estimates of annual U.S. mortality from wind turbine collisions range between 20,000 and 573,000 birds (Erickson et al., 2001, Erickson et al., 2005, Manville, 2009, Sovacool, 2012, Smallwood, 2013). Earlier estimates were generated by summarizing a small sub-set of industry reports and extrapolating mortality rates across all turbines (Erickson et al., 2001, Erickson et al., 2005), by using small samples of preliminary data (Sovacool, 2012), or by using undocumented methods (Manville, 2009). A recent study estimates annual U.S. collision mortality at 573,000 birds and greatly improves upon earlier efforts by using data from a large sample of wind facilities and by accounting for several methodological differences among the studies used (Smallwood, 2013). However, this study did not distinguish between lattice and monopole turbines. Because monopole turbines comprise the vast majority of all installed U.S. wind turbines, it is important to separately estimate mortality and assess correlates of mortality for this turbine type.
We reviewed the wind energy literature, including both peer-reviewed articles and unpublished industry reports, and extracted data to systematically estimate bird collision mortality and mortality correlates at monopole turbines in the contiguous U.S. Specifically, we (1) defined inclusion criteria to ensure a baseline level of rigor for studies used in the estimate, (2) fitted a predictive model that includes correlates of mortality, accounts for differences among studies in the proportion of the year during which collision events were sampled, and includes estimate uncertainty, and (3) implemented the fitted model to estimate bird collision mortality for wind facilities in the contiguous U.S.
Section snippets
Literature search
We searched Google Scholar, the Web of Science database (using the Web of Knowledge search engine), and the National Renewable Energy Laboratory’s Wind-Wildlife Impacts Literature Database (http://wild.nrel.gov/) to identify studies documenting bird collisions with wind turbines. We also searched Google because most industry reports are not indexed in databases. We used the search terms “bird AND wind turbine” with “collision,” “mortality,” “fatality,” “carcass,” and “post-construction”; all
Correlates of mortality
The additive 2-variable model that included turbine hub height and region was the most strongly supported model in our analysis, followed by the multiplicative height-region model. Because the relative strength of support was greater for the additive model (Table 1), we used this model for mortality prediction. The univariate region and hub height models each received less support than the 2-variable models; however, because both variables were included in the best-supported model, we compared
Comparison to other mortality estimates
Our mean projected estimate of 234,012 annual bird collisions in the contiguous U.S. – and even our low-end estimate (140,438) – is greater than most previous estimates, including ∼20,000 birds/yr (Sovacool, 2012), 10,000–40,000 birds/yr (Erickson et al., 2001, Manville, 2005), and 20,000–40,000 birds/yr (Erickson et al., 2005). Two recently published annual estimates exceed our upper estimate of 327,586 birds: 440,000 (Manville, 2009) and 573,000 (Smallwood, 2013). We provide the first
Role of the funding source
The funder had no role in study design; collection, analysis, and interpretation of data; in writing the report; and in the decision to submit the paper for publication.
Acknowledgments
We thank the following people and organizations for facilitating or providing access to unpublished studies: T. Bartunek, J.W. Demastes, K. Fuller, P. Kerlinger, K. Kronner, C. Machtans, D. Mason, T. Sandberg, G.D. Schnell, BHE Environmental, Curry and Kerlinger LLC., Iberdrola Renewables, and the National Renewable Energy Laboratory-National Wind Technology Center. We also thank T. Alison, J. Berry, M.M.P. Huso, D.H. Johnson, T. Longcore, A. Manville, M. Parr, and A.C. Peterson for discussions
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