Re-assessing the effectiveness of wire-marking to mitigate bird collisions with power lines: A meta-analysis and guidelines for field studies

The expansion of overhead power lines worldwide challenges companies of energy transmission and distribution, regulators and environmental consultants, among other stakeholders, to effectively mitigate their negative impacts on wildlife. Wire-marking is currently the most widespread and recommended measure to reduce bird collisions with these infrastructures. Nevertheless, and despite its importance for a bird-friendly development of energy projects, there is still much uncertainty about what explains wire-marking effectiveness. We performed an extensive literature review and meta-analysis to evaluate the overall effectiveness of wire-marking in reducing bird collisions with power lines, including the possible influencing factors of power line voltage, habitat and type of device. We gathered data from 35 field studies across the world (which included 66 trials) assessing the effectiveness of wire-marking based on regular carcass searches beneath power lines. Overall, wire-marking reduced bird collisions with power lines by half (50.4%; 95% Confidence Interval Estimate: 40.4-58.8%), although this estimate of effectiveness is lower than the one reported in a meta-analysis performed in 2011. Despite the effort to include both peer-reviewed and grey literature studies in the present meta-analysis, the risk of publication bias could not be entirely excluded and may be still overestimating the true overall effect of wire-marking. High heterogeneity among the study outcomes hindered the power to detect clear moderating effects, with none of explanatory variables being statistically significant. Large between-study heterogeneity is (to some extent) explained by the variety of anti-collision devices available, wire-marking intensities used and ecological circumstances in which the experiments were carried out. Nonetheless, it may be also related to within-study methodological biases and reporting gaps in the existing field studies. Robust experimental designs (ideally using Before-After-Control-Impact approaches), comprehensive reporting of results and broad dissemination of study findings are needed to increase the statistical power of future meta-analyses. Ways to achieve these improvements in field studies are presented in detail. Their inclusion in future meta-analyses will increase the knowledge on the drivers of wire-marking effectiveness, which is critical to better inform decision-making processes and environmental management plans.