Emerging technologies and strategies offer the opportunity to minimize or avoid collision fatalities among birds and bats at operating wind turbines. Deterrence technologies and strategies aim to actively discourage birds or bats from entering the rotor-swept “risk zone” (the area of the spinning turbine blades) using visual or auditory cues or features designed either to help the bird or bat perceive and avoid the risk, or simply to repel it from approaching too close. Cues may be ongoing or activated only when an animal is detected near the risk zone, and they must be something the target animal will both perceive and respond to. For example, what might work to deter eagles may not be effective for another type of bird, and what works for one species of bat may not work equally well for another species. Understanding the behaviors that put certain birds and bats at higher risk of collision with turbines is in developing effective minimization strategies and is therefore an ongoing area of research.
Ultrasonic and audible deterrents
Ultrasonic transmitters have been developed with the aim of deterring bats from entering the rotor-swept area by creating a sonic field with frequencies that confound the bat’s use of echolocation. Experimental trials have shown that ultrasonic devices can reduce bat activity and foraging success, and evaluation of similar devices installed on wind turbines has shown some reduction in fatalities for some species relative to control turbines.
Auditory warning signals appear to deter raptors from approaching wind turbines. One such system includes a speaker that emits a warning sound when a target species is detected within a pre-defined distance of a turbine, and then emits a stronger dissuasion sound if the bird continues to approach the turbine. Field tests found that the deterrence rate for golden eagles in response to the warning/dissuasion signals was between 52% and 83%. Given that birds and humans share similar hearing frequencies, use of avian audible deterrents may be limited in areas near people.
Visual deterrents to reduce risk for birds
Efforts to make turbine blades more visible to birds have been proposed as a way to reduce bird collision risk, but specific methods for increasing turbine visibility have shown limited success in reducing collision fatalities. For example, it has been hypothesized that towers and blades coated with ultraviolet (UV) paint may be more visible to birds, making them easier to avoid. In the only known test, researchers compared fatality rates at turbines with UV paint to turbines coated with standard paint and found no difference. Several raptor species have shown little response to UV light.
In a recent study, researchers tested whether painting one of three turbine blades black would increase visibility of moving turbine blades to birds, thereby reducing bird collisions. The study was conducted using a small number of turbines at the Smøla wind power plant in Norway, which had been previously surveyed for bird carcasses. The annual fatality rate for all birds was reduced by over 70% at the four turbines with a painted blade when compared to the neighboring control turbines. Effects on individual species could not be evaluated statistically; however, painting blades may have reduced white-tailed eagle collisions, which had no fatalities recorded after blade painting. Additional studies are underway at other wind energy projects to further test this approach to reducing bird collision fatalities.
Visual deterrents to reduce risk for bats
Some scientists have proposed that using UV light to illuminate turbines without presenting a visual disturbance to humans or other animals may provide a deterrent for bats that can perceive UV light – either by enabling bats to better distinguish turbines from potential foraging and roosting habitat, or simply by discouraging them from approaching a lighted area. Bats generally tend to avoid lighted areas, and one study of hoary bats, the most commonly observed bat fatality, found that illuminating otherwise attractive tree habitat with dim UV lighting decreased activity of this species around those trees. However, a study led by U.S. Geological Survey found evidence that bats may actually have been attracted to turbines illuminated with dim UV light at the National Wind Technology Center in Colorado.
Other research has evaluated whether bats are attracted by the smooth reflective surfaces of turbine towers, which they could perceive as water and which also provides a clear ultrasonic background for hunting insects using echolocation. However, one study comparing bat activity at turbine towers that had been painted with a textured coating and at smooth, untextured towers found no evidence that textured towers were avoided by bats.