Bird mortality due to collisions with wind turbines is one of the major ecological concerns associated with wind farms. Data on the factors influencing collision risk and bird fatality are sparse and lack integration. This baseline information is critical to the development and implementation of effective mitigation measures and, therefore, is considered a priority research topic. Through an extensive literature review (we compiled 217 documents and include 111 in this paper), we identify and summarize the wide
range of factors influencing bird collisions with wind turbines and the available mitigation strategies.
Factors contributing to collision risk are grouped according to species characteristics (morphology, sensorial perception, phenology, behavior or abundance), site (landscape, flight paths, food availability and weather) and wind farm features (turbine type and configuration, and lighting). Bird collision risk results from complex interactions between these factors. Due to this complexity, no simple formula can be broadly applied in terms of mitigation strategies. The best mitigation option may involve a combination of more than one measure, adapted to the specificities of each site, wind farm and target species. Assessments during project development and turbine curtailment during operation have been presented as promising strategies in the literature, but need further investigation. Priority areas for future research are: (1) further development of the methodologies used to predict impacts when planning a new facility; (2) assessment of the effectiveness of existing minimization techniques; and (3) identification of new mitigation approaches.
Future research: what is left to understand
Nowadays, wildlife researchers and other stakeholders already have a relatively good understanding of the causes of bird collisions with WT. Through our extensive literature review, we have been able to identify some of the main factors responsible for this type of fatality and acknowledge the complexity of the relationships between them.
From the factors described in Section 3, we find that lighting is the one least understood, and further studies should address this topic by testing different lighting protocols in WT and their effects on bird fatalities, with a special focus on migratory periods during bad weather conditions.
We also anticipate that the expansion of WF to novel areas (with different landscape features and bird communities) or innovative turbine technologies may raise new questions and challenges for the scientific community. This is currently the case for offshore developments. To date, the main challenge in offshore WF has been the implementation of a monitoring plan and making accurate predictions of collision risk due to the several logistical constraints. The major constraints include assessing accurate fatality rates, as it is not possible to perform fatality surveys, and studying bird movements and behavior at an offshore WF, since this usually implies deployment of automatic sampling devices, such as radar or camera equipment (e.g. Desholm et al., 2006).
Due to the complexity of factors influencing collision risk, mitigating bird fatality is not a straightforward task. Mitigation should therefore be a primary research area in the near future. As species-specific factors play an important role in bird collisions, specialists should ideally strive to develop guidance on species-specific mitigation methods, which are still flexible enough to be adaptable to the specificities of each site and WF features.
Appropriate siting of WF is still the most effective measure to avoid bird fatalities. Since there are no universal formulas to accomplish this, it is essential to fully validate the methodologies used to predict impacts when planning a new facility and when assessing the environmental impact of a forthcoming project. In this context, comparing prior risk evaluations with the fatalities recorded during an operational phase should be a priority.
In many cases, pre-construction assessments may be sufficient to prevent high bird fatality rates but in others, it will be essential to combine this approach with different minimization techniques.
Political and public demand for renewable energy may prompt authorities and wind energy developers to implement WF in areas that pose risks to birds. In these cases, minimization techniques are a crucial element for limiting bird fatalities.
In this context, the development of efficient mitigation techniques that establish the best trade-off between bird fatality reduction, losses in energy production and mplementation costs is a high priority. Although turbine shutdown on demand seems to be a promising minimization technique, evidence of its effectiveness in different areas and for different target species is lacking. In addition, research should also focus on other options, as in certain situations less demanding approaches may also achieve positive results.
It is also important to ensure that the monitoring programs apply well designed experimental designs, for example a Before-After-Control Impact (BACI) approach (Anderson et al., 1999; Kunz et al., 2007; Strickland et al., 2011). BACI is assumed to be the best option to identify impacts, providing reliable results. However, some constraints have been identified and there are several assumptions that need to be fulfilled to correctly implement these types of studies (see Strickland et al. (2011) for a review on experimental designs).
Finally, it is important to ensure that monitoring programs are implemented and that they provide robust and comprehensive results. Also, monitoring programs results, both on bird fatalities and the effectiveness of the implemented mitigation measures, should be published and accessible, which is not always the case (Subramanian, 2012) Sharing this knowledge will facilitate the improvement of the mitigation hierarchy and the development of WF with lower collision risks.