Flight characteristics forecast entry by eagles into rotor-swept zones of wind turbines. Rolek, B. W., Braham, M. A., Miller, T. A., Duerr, A. E., Katzner, T. E., McCabe, J. D., Dunn, L. & McClure, C. J. 2022. IBIS. DOI: 10.1111/ibi.13076. VIEW

To mitigate the risk of wildlife collisions, wind turbines can be either stopped or slowed down (Allison et al. 2017). However, an operator needs to be sure that the approaching bird will actually enter the area swept by the turbine blades before shutting down the system. Unnecessary shutdowns lead to significant energy loss. Despite current monitoring systems, false alarms are still prevalent. For instance, the Top of the World Power Facility in Wyoming (USA) unnecessarily curtailed its operations 70% of the time, as the approaching birds never entered the rotor-swept zone (McClure et al. 2021). Clearly, we need better detection systems to balance collision risk with power generation.

Flight altitude
In a recent study, Brian Rolek and his colleagues examined the flight paths of Bald Eagles (Haliaeetus leucocephalus) and Golden Eagles (Aquila chrysaetos) around the Top of the World Power Facility in Wyoming. Based on a statistical model, they were able to link certain flight characteristics to the probability of an eagle entering the rotor-swept zone. Eagles flying at an altitude of approximately 89 meters showed the highest likelihood of traversing the wind turbine blades. This result aligns with the fact that the turbines in Wyoming are 80 meters tall, with the blades extending to a maximum height of 131 meters. Nevertheless, this finding highlights the importance of taking into account flight altitude in curtailment criteria.

Figure 1. Eagles were more likely to enter the rotor-swept zone when flying at an altitude of about 89 meters (figure a) Moreover, slow-flying eagles showed a higher probability of entering the zone compared to fast-flying eagles (figure b).

Approach speed
In addition to flight altitude, the researchers observed an intriguing relationship between flight speed and the probability of eagles entering the rotor-swept zone. Slow-flying eagles exhibited a higher probability of entering the zone compared to their fast-flying counterparts. This discrepancy can probably be explained by behavioural differences. Fast-flying eagles focus on forward navigation while surveying the landscape (Johnston et al., 2014), enabling them to easily avoid wind turbines. In contrast, slow-flying eagles are likely engaged in hunting activities, scanning the ground for potential prey (Smallwood et al., 2009). Consequently, they are less likely to detect the presence of wind turbines.

The identified flight characteristics – including altitude and speed – allowed the researchers to successfully predict the entry of eagles into the rotor-swept zone in Wyoming. Moreover, the model forecasted eagle entry at another location (the Manzana Wind Power Project), suggesting that this approach can be widely applicable. With the implementation of such detection methods, we can strive to protect eagles from collisions while simultaneously generating sufficient green energy.


Allison, T.D., Cochrane, J.F., Lonsdorf, E. & Sanders-Reed, C. (2017). A review of options for mitigating take of Golden Eagles at wind energy facilities. Journal of Raptor Research 51: 319– 333. VIEW

Johnston, N.N., Bradley, J.E. & Otter, K.A. (2014). Increased flight altitudes among migrating golden eagles suggest turbine avoidance at a rocky mountain wind installation. PLoS One 9, e93030. VIEW

McClure, C.J.W., Rolek, B.W., Braham, M.A., Miller, T.A., Duerr, A.E., Mccabe, J.D., Dunn, L. & Katzner, T.E. (2021). Eagles enter rotor-swept zones of wind turbines at rates that vary per turbine. Ecology and Evolution 11: 11267– 11274. VIEW

Smallwood, K.S., Rugge, L. & Morrison, M.L. (2009). Influence of Behavior on Bird Mortality in Wind Energy Developments. Journal of Wildlife Management 73: 1082– 1098. VIEW

Image credits

Top right: Bald Eagle (Haliaeetus leucocephalus) | Peter K Burian | CC BY-SA 4.0 Wikimedia Commons

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