Comparing Field Methods Used to Determine Bird and Bat Fatalities

Abstract

Wind energy production across the globe has grown over the years and Portugal is no exception. Like other forms of energy production, wind energy causes ecological impacts and these impacts are a matter of major concern. One of the impacts caused by operating wind farms is bird and bat casualties due to collision with the wind turbines. Since real mortality is very difficult to determine, due to the impossibility of detecting all carcasses and the probability of carcass disappearance before being detected, field methods for collecting data and estimating mortality have been implemented. Regular searches around wind turbines are implemented to detect carcasses. The protocol of searches must define the size of the searched area, the search method, the duration and frequency of the searches according to guidelines and the objectives of each study. To correct the observed mortality and estimate real mortality, correction factors, such as the probability of carcass persistence or the carcass detection rate, must be assessed by field trials. For Portugal, a search area of 5 m more than blade length and random walk or zig-zag methods resulted in a cost-effective way to collect carcasses. A frequency of 1–3 days is recommended in order to reduce the bias of mortality estimates. If a considerable number of removal trials had to be carried out, camera-trapping was revealed to be a cost-effective method. Otherwise, a carefully planned traditional method also obtained accurate estimates. For detection trials, the use of models is recommended and the number of models and the design should be adapted to homogeneity or heterogeneity of vegetation cover.