Estimating bird and bat fatality at wind farms: a practical overview of estimators, their assumptions and limitations

Over the last few years, great efforts have been made to improve the methodologies used to assess bird and bat fatalities at wind farms. For that purpose, several mortality estimators have been proposed. In general, these estimators account for: 1) partial coverage; 2) carcass removal (e.g. by scavengers or decay); and 3) imperfect detection. Perhaps surprisingly, a universal estimator that ensures good quality estimates under general circumstances is still lacking. Further, the existing estimators include different adjustment approaches and a variety of often implicit and misunderstood assumptions that may not be valid, making it difficult for practitioners to choose between them. Focusing on bird and bat fatality at onshore wind farms, we summarise and discuss implementation aspects and the assumptions involved in seven commonly used estimators. This should provide researchers requiring these methods with a basis to choose the most appropriate estimator under a given set of conditions, and contribute to increased standards in wind farm wildlife fatality estimation.