An estimator of wildlife fatality from observed carcasses

Abstract

Counts of animal carcasses are often used to estimate fatality caused by disease, environmental accidents (oil spills, radiation leaks), or human structures (power lines, sky scrapers, wind turbines). The need to adjust raw carcass counts for imperfect detectability to produce unbiased estimates of fatality has long been recognized, but the accuracy and precision of some estimators used to make the adjustments have not been evaluated. In this paper, I formalize a conceptual model of fatality and the factors that lead to imperfect detection, primarily removal by scavengers before searches can be carried out and inability of searchers to find all remaining carcasses. I propose an estimator of fatality that adjusts for imperfect detectability. Through simulation I evaluate the statistical properties (bias and precision) of this estimator and two others commonly used to estimate fatality at wind power facilities, when sources and magnitudes of imperfect detectability vary. None of the estimators was always unbiased under all conditions. Bias in the proposed estimator never exceeded W27% whereas bias in the other two estimators was always negative and exceeded that of the proposed estimator in 98% and 93% of the simulated conditions, respectively. The proposed estimator was relatively robust to variation in sources and magnitudes of imperfect detectability, but was sensitive to distributional assumptions regarding carcass removal rates and searcher efficiency. It offers significant improvement over two current estimators and provides relatively unbiased estimates of fatality that can be applied under a variety of conditions and survey protocols.

A Forensic Investigation Into the Etiology of Bat Mortality at a Wind Farm: Barotrauma or Traumatic Injury?

“…Lung, silver-haired bat (Lasionycteris noctivagans); salvaged from a recently argued that barotrauma may also result in sublethal injuries, such as hearing impairment and other internal injuries, that would allow bats to move away from turbines before debilitation and death. 40 If fatality estimators 22,40 were adjusted to account for such presumed off-site deaths due to barotrauma, total fatality estimates would significantly increase. Fatality rates are not trivial, as they are used in the postconstruction risk assessment of WFs to evaluate for negative impacts and determine whether mitigation measures are needed.…”

Impacts of Wind Energy Development on Bats: A Global Perspective

“…We present information on estimates of bat fatalities as reported in published literature or publically available reports, but caution that studies had varying levels of effort, used different estimators (e.g., Huso 2011;Korner-Nievergelt et al 2013) and different methods to quantify bias Strickland et al 2011), thus biasing estimates. Also, most estimators fail to adequately account for unsearched area near turbines (Huso and Dalthorp 2013), which further biases estimates.…”

Mortality of Selected Avian Orders Submitted to a Wildlife Diagnostic Laboratory (Southeastern Cooperative Wildlife Disease Study, Usa): A 36-Year Retrospective Analysis

“…Over the years several estimators have been proposed (e.g. Johnson et al 2003, Erickson et al 2004, Kerns et al 2005, Jain et al 2007, Huso 2010, Korner-Nievergelt et al 2011, mainly for wind power facilities but these can be applied to other human infrastructures. However, some of these mortality estimators are complex, requiring, in some cases, a great effort and statistical expertise from the technician who is conducting the data analysis.…”