2023

Bird mortality by collision is one of the major biodiversity impacts of transmission power lines. The European Union legislation determines that overhead power lines subject to Environmental Impact Assessment (EIA) procedure should be monitored at post-construction to confirm the predicted impacts, and evaluate the effectiveness of the implemented mitigation measures. In light of the frequent calls to improve study design and quality of field data in EIA context, we reviewed a decade (2004–2015) of post-construction monitoring practices adopted in Portugal for assessing the impacts caused by transmission lines (150–400 kV) on birds and evaluating wire-marking effectiveness to reduce collisions, the main mitigation measure. We reviewed 31 monitoring programs to (i) characterize the practices (field surveys and its methods) adopted, (ii) identify specific objectives behind field surveys, (iii) detect the main methodological limitations, and (iv) provide guidelines to improve future bird monitoring programs.

Overall, reviewed studies contained significant field efforts, always including bird carcass surveys (very often with trials to assess carcass detection and persistence biases) to estimate mortality rates and often including surveys to determine bird abundance and the frequency of flights crossing the wires. However, we also found limitations, namely (i) a frequent lack of clear reporting of specific objectives behind field surveys, hindering the usefulness of data collected, (ii) a dominance of poor methodological approaches evaluating indirect impacts and wire marking effectiveness, and (iii) the (less frequent) use of inadequate protocols and a lack of standardization, hindering comparability across studies. To overcome these limitations, we propose a methodological framework and specific recommendations to improve current practices for measuring the impacts of new transmission lines on birds and evaluating the effectiveness of wire-marking to reduce collisions. Although developed for the Portuguese EIA context, these recommendations are likely applicable to many other countries.