A Collision Risk Model to Predict Avian Fatalities at Wind Facilities: An Example Using Golden Eagles, Aquila chrysaetos

Abstract

Wind power is a major candidate in the search for clean, renewable energy. Beyond the technical and economic challenges of wind energy development are environmental issues that may restrict its growth. Avian fatalities due to collisions with rotating turbine blades are a leading concern and there is considerable uncertainty surrounding avian collision risk at wind facilities. This uncertainty is not reflected in many models currently used to predict the avian fatalities that would result from proposed wind developments. We introduce a method to predict fatalities at wind facilities, based on pre-construction monitoring. Our method can directly incorporate uncertainty into the estimates of avian fatalities and can be updated if information on the true number of fatalities becomes available from post-construction carcass monitoring. Our model considers only three parameters: hazardous footprint, bird exposure to turbines and collision probability. By using a Bayesian analytical framework we account for uncertainties in these values, which are then reflected in our predictions and can be reduced through subsequent data collection. The simplicity of our approach makes it accessible to ecologists concerned with the impact of wind development, as well as to managers, policy makers and industry interested in its implementation in real-world decision contexts. We demonstrate the utility of our method by predicting golden eagle (Aquila chrysaetos) fatalities at a wind installation in the United States. Using pre-construction data, we predicted 7.48 eagle fatalities year-1 (95% CI: (1.1, 19.81)). The U.S. Fish and Wildlife Service uses the 80th quantile (11.0 eagle fatalities year-1) in their permitting process to ensure there is only a 20% chance a wind facility exceeds the authorized fatalities. Once data were available from two-years of post-construction monitoring, we updated the fatality estimate to 4.8 eagle fatalities year-1 (95% CI: (1.76, 9.4); 80th quantile, 6.3). In this case, the increased precision in the fatality prediction lowered the level of authorized take, and thus lowered the required amount of compensatory mitigation.

Predicting eagle fatalities at wind facilities

“…The Eagle Conservation Plan Guidance (USFWS : appendix D [table D‐1]) and New et al () modeled annual eagle fatalities ( F ) caused by collisions with wind turbines using a CRM as: where ϵ is an expansion factor that scales the fatality rate based on the size of the project to the annual predicted fatalities for the project, λ is the rate of eagle exposure to turbine hazards, and the collision rate ( C ) is the rate that eagle exposure results in a collision with a turbine. The CRM is a combination of 2 separate distributions: the distribution for the collision rate and the distribution for exposure rate, and a constant associated with facility characteristics (i.e., the expansion factor).…”

“…Exposure rate (λ) is the expected number of exposure events (i.e., eagle min/(hr × km 3 ); New et al ) across the facility. The USFWS recommends estimating exposure events using the number of minutes eagles are observed during survey times within 800 m of point count locations and flying no higher than 200 m, assuming high detection rate within this area (USFWS ).…”

“…A formal policy of adaptive management has been adopted by the USFWS (Walters ), in which key uncertainties regarding the impact on golden eagles by wind facilities are to be minimized through a sequential process of model development, model testing and comparison, accumulation of updated information, and new data, followed by model re‐building and renewed model testing. New et al () recently published an update to the prior probability distribution for exposure rate to be used in the CRM, demonstrating a pathway for providing updates so that the information is available to managers, statisticians, and scientists.…”

The utility of point count surveys to predict wildlife interactions with wind energy facilities: An example focused on golden eagles

“…The modified point count approach recommended by the USFWS is used to record the amount of time that eagles spend in a three-dimensional survey plot. These data are then input to an eagle risk model (New et al, 2015) to predict eagle exposure to turbines, collision probability and fatality rates for a proposed wind facility (e.g., Douglas et al, 2012). However, it is not clear how accurately the data collected during these point counts relate to actual use of the project footprint by eagles.…”

“…668 et seq.). Consequently, substantial effort has been dedicated to understand and mitigate threats to this species and, at wind energy facilities, detailed protocols have been designed to predict and manage disturbance and take of golden eagles (New et al, 2015;Strickland et al, 2011;USFWS, 2013). However, golden eagles are not easy to monitor.…”