Chapter 4: Minimizing Collision Risk to Wildlife During Operations

Minimization: Curtailment

Updated December 27th, 2022

Birds and bats are at risk of collision with operating wind turbines, but curtailment strategies can provide a solution for minimizing risk from wind energy operation for some species. Curtailment is the feathering of wind turbine blades (angling the blades parallel to the wind to slow or stop them from turning) when risk of collision is determined to be high. This strategy has proven to be effective at reducing bat collision fatalities. It can also be effective in some instances to avoid fatalities of larger birds, such as eagles.

Curtailment to reduce risk for bats

Raising the cut-in speed

Most bat fatalities occur at low wind speeds. Increasing the cut-in speed (wind speed at which the turbine starts producing energy) to higher wind velocities has been shown to substantially reduce bat fatalities. The manufacturer’s set cut-in speed for most contemporary turbines is between 3.0 and 4.0 meters/second (m/s.) An examination of ten separate studies showed reductions in bat fatalities ranging from 50 to 87% at sites that curtailed blade rotation at low wind speeds – e.g. below 5 or 6 m/s – when compared to normally operating turbines. The American Wind Energy Association (now the American Clean Power Association) announced a voluntary industry-wide best management practice in 2015 of curtailing turbines at low wind speeds through the fall bat migration season. The adoption of the protocol reflects more than a decade of scientific collaboration between the industry and conservationists.

Blanket curtailment

One curtailment strategy to reduce risk to bats is blanket curtailment, which refers to consistent application of curtailing turbines below a certain wind speed without consideration of other site-specific information. Typically, blanket curtailment is implemented beginning in mid-summer when bat fatalities begin to increase and continues through the end of fall migration. Some states compel blanket curtailment in the spring and early summer months as well.

Smart curtailment

While blanket curtailment can be an effective way to reduce bat collision fatalities, it can result in unnecessary loss in electricity production and does not discriminate for species that are most at risk, or the conditions under which they are most at risk. Emerging research is helping identify conditions when bat collision risk is elevated, making it possible to optimize curtailment to be effective at reducing fatalities while minimizing power loss. For example, a smart curtailment approach that combined real-time data on wind speed and bat activity near turbines reduced estimated fatalities of all bats at a facility by nearly 85% while reducing the overall curtailment time by nearly 50% relative to controls.

Curtailment to reduce risk for birds

Informed curtailment: Monitoring and selective shut-down

Selective shutdown of turbines associated with higher risk may be an effective strategy for reducing fatalities of some raptor species. Some of the highest raptor fatality rates have been observed in southern Spain where raptors congregate to cross the Strait of Gibraltar to Africa during migration. One study reported an average of more than 50% reduction in griffon vulture fatalities at a facility due to selective shutdown of high-fatality rate turbines when vultures were observed near them by human observers.

In some situations, such as expansive project areas, more complex or difficult to access terrain, or nocturnal migration events, radar may complement or even substitute for human observers. However, in situations where species identification is critical to determining whether and when to curtail turbines, radar technology is ineffective. It’s also unclear whether curtailment is an effective strategy to reduce collision risk for nocturnally migrating birds, given that songbirds collide frequently with immobile buildings.

Emerging technologies using machine learning and artificial intelligence may be able to bridge the gap. For example, an automated system can successfully detect and classify eagles in the vicinity of a wind project, can detect large birds at far greater distances than human observers, and can automatically curtail turbines. A recent study found this automated system successfully reduced eagle fatalities by 85% at the treatment site relative to the control site. Researchers are also investigating the use of telemetry to trigger curtailment for vulnerable focal species near wind energy facilities.