Understanding the causes of why bats approach and interact with wind turbines may provide more efficient means of monitoring and mitigating impacts (Cryan and Barclay 2009). Mortality monitoring continues to be a useful method to quantify the impact of wind turbines on bats. However, searching for carcasses can be expensive to implement, and the results lack the temporal scale necessary to determine when and under what conditions collisions occur. Moreover, standard mortality monitoring conducted at land-based wind energy facilities is not possible for offshore wind energy development. Precise data on the behavior bats exhibit near wind turbines and on collisions are necessary to develop and advance strategies that meet renewable energy generation and conservation goals.
There is increasing evidence that some species of bats are attracted to wind turbines. Studies using thermal videography have documented bats approaching the tower, nacelle, and blades (Horn et al. 2008, Ahlén et al. 2009, Cryan et al. 2014). Several plausible attraction hypotheses have been introduced but none have been confirmed or disproven with conclusive evidence (Kunz et al. 2007, Cryan and Barclay 2009), in part because it is challenging to detect, observe and track bat movements. In addition, there is limited information on basic bat behavior and physiology to understand what drives their responses to environmental cues. These responses may vary by species, landscape or habitat conditions, and turbine dimensions, or interact with one another in unpredictable ways.
Advancing research to monitor bat interactions with wind turbines can improve technology-based monitoring approaches, which may be more cost-effective for land-based and essential for offshore wind energy development. It can also provide information which may improve minimization strategies. For example, the specific timing and conditions when collisions occur may reduce the amount of time curtailment is implemented or provide information on how bats respond to different deterrent technologies. Advances in technologies such as thermal cameras, acoustic detectors, telemetry, and radar combined with innovations in machine learning and artificial intelligence makes studying bat behavior at wind turbines increasingly feasible.
As research progresses, it is important to articulate the goals that will enhance our understanding of how bats perceive wind turbines. At the Bats and Wind Energy Cooperative (BWEC) 2018 Science Meeting, several priorities for understanding bat behavior at the wind-turbine or wind energy-facility scales were established (BWEC 2019). One of these priorities was to develop a list of research questions for behavioral studies. The need to better understand bat behavior was reiterated at the State of the Science and Technology for Minimizing Impacts to Bats from Wind Energy workshop, convened in November 2019 (Hein and Straw 2021). In January 2020, the National Renewable Energy Laboratory, on behalf of the BWEC, organized a forum with subject matter experts (Table 1) to discuss next steps for research pertaining to bat and wind turbine interactions.
The participants discussed several research questions related to the available technology used to study bat behavior, the behavior of bats flying near wind turbines, the conditions when collisions occur, and improving the effectiveness of minimization strategies. Most activities presented below are centered at the turbine-scale, though some may require facility-scale monitoring. Some of these data may already be available from previous studies but can be analyzed differently (e.g., using more reliable machine-learning algorithms) or combined with other datasets to increase samples size. For new studies, it is important to select sites with greater species diversity, and relatively high bat activity and mortality.
The participants primarily focused on the use of thermal cameras as the most practical technology for recording bat interactions at the turbine-scale. Other technologies, such as acoustic detectors, telemetry, and radar, can be used in combination with cameras to provide additional information. During the expert forum, research questions regarding equipment and analysis tools, behavior, mortality, and evaluating minimization strategies were transformed into four broad research goals:
- Advance technology, methods, and analysis tools for behavioral research studies,
- Characterize bat behavior at the wind-turbine-scale,
- Characterize bat collision with wind turbines (e.g., time of night, conditions, and location along the blades where collisions occur), and
- Evaluate the effectiveness of strategies to reduce bat mortality at wind turbines.
The participants did not rank goals by species, priority, or timeline as this likely varies by region, facility, and stakeholder group. For example, efforts to understand the behavior of threatened and endangered species, or those being considered for listing under the Endangered Species Act, may take precedence over non-listed migratory tree-roosting bats in certain scenarios, whereas reducing mortality of migratory tree-roosting bats may be a priority in other situations. The goals, subgoals, and activities are individually described, but several can be incorporated in any study or addressed simultaneously. Examples include:
- Species identification: The need to assess species-specific differences is essential to differentiate behaviors among species that are vulnerable to collisions or the presence of threatened or endangered species. Identifying species or species groups is listed as a subgoal but should be a component of all studies.
- Assessing behavior and mortality: Research focused on behavior is separate from those centered on mortality, but in some situations, both can be examined during the same study. Moreover, when evaluating the effectiveness of a minimization strategy, information on behavior can be simultaneously collected to provide greater insight as to how bats are responding to the strategy.
- Combining multiple technologies: Although each technology provides useful information, there are limitations and biases associated with their use. Pairing technologies, such as acoustic detectors and thermal cameras, can help to overcome the constraints of an individual technology.
- Capturing individuals: Several activities may require the capture of individuals, which provides the opportunity to secure radio- or GPS-tags. The live capture of bats also provides an opportunity for taking blood and tissue samples (e.g., biopsy, hair, or guano), which may help examine physiological aspects associated with behavior and risk.