Abstract

Estimating bird and bat mortality at wind facilities typically involves searching for carcasses on the ground near turbines. Some fraction of carcasses inevitably lie outside the search plots, and accurate mortality estimation requires accounting for those carcasses using models to extrapolate from searched to unsearched areas. Such models should account for variation in carcass density with distance, and ideally also for variation with direction (anisotropy). We compare five methods of accounting for carcasses that land outside the searched area (ratio, weighted distribution, non-parametric, and two generalized linear models (glm)) by simulating spatial arrival patterns and the detection process to mimic observations which result from surveying only, or primarily, roads and pads (R&P) and applying the five methods. Simulations vary R&P configurations, spatial carcass distributions (isotropic and anisotropic), and per turbine fatality rates. Our results suggest that the ratio method is less accurate with higher variation relative to the other four methods which all perform similarly under isotropy. All methods were biased under anisotropy; however, including direction covariates in the glm method substantially reduced bias. In addition to comparing methods of accounting for unsearched areas, we suggest a semiparametric bootstrap to produce confidence-based bounds for the proportion of carcasses that land in the searched area.

Highlights

  • Impacts to birds from collisions with wind turbines have been of concern in the U.S since the early 1990s (Orloff and Flannery 1992) and impacts to bats since the early-2000s (Kerns and Kerlinger 2004)

  • In the description of some methods below, and in the results presented in Sect. 4, we assume the conditional probability, G, that a carcass is observed given it landed on roads and pads (R&P) is known

  • We compared the methods via a simulation study that considered different R&P configurations, spatial carcass distributions, and average carcass per turbine rates

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Summary

Introduction

Impacts to birds ( raptors) from collisions with wind turbines have been of concern in the U.S since the early 1990s (Orloff and Flannery 1992) and impacts to bats since the early-2000s (Kerns and Kerlinger 2004). PCM to estimate mortality—the total number of turbine-induced bird or bat fatalities occurring over a specified period of time at individual wind projects—typically involves conducting carcass surveys, where human observers (or well-trained human–dog teams) search for carcasses within plots centered at randomly selected turbines (US Fish and Wildlife Service 2012). Trials designed to estimate the probability that a carcass persists to the search must be flexible enough to allow for non-constant removal and must account for potential differences due to carcass size, season, vegetation density in which the carcass lands, and other carcass-specific factors (Bernardino et al 2011; Bispo et al 2013; Turner et al 2017). Placing 50 trial carcasses, as recommended by Strickland et al (2011), in each of 4 size classes, 4 visibility/vegetation classes, and 4 seasons would result in 3200 total carcasses during a year and could potentially attract scavengers to the site

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