Chapter 2: What Do We Know about Impacts & Risk Factors to Wildlife and Habitat?

Collision Risk and Risk Factors

Updated September 01, 2021

How do we measure collision impacts and assess risk for birds and bats at wind energy facilities?

Conducting structured searches for bird and bat carcasses beneath operational wind turbines is our main source of information about collision impacts. These post-construction monitoring, or “PCM,” studies are designed to estimate bird and bat fatality rates for a given wind project; collectively, PCM study results have provided a basis for estimating collision fatalities across all wind energy projects and help to shed light on patterns of collision risk and risk factors for bird and bat species.

Study protocols

The protocol – that is, objectives, design, and methodology – used for PCM studies includes the number of turbines to monitor, the delineation of search plots and how searchers cover them, and the interval of time between searches. Study design takes into account topography and types of land cover that may affect searcher access and the visibility of carcasses, seasonal activity, landcover found within a project area, and the interval between carcass searches. For each carcass observed, searchers typically record the date, species, turbine being searched and the distance of the carcass to that turbine, GPS coordinates, vegetation cover, and condition of the carcass. See the U.S. Fish & Wildlife Service’s Land-based Wind Energy Guidelines, pp. 34-39, for a more detailed description of the questions to be answered and the methods and metrics to be used in PCM (Tier 4a) studies.

Technologies that may supplement or replace fatality searches

Technologies such as cameras or impact sensors are being tested to supplement – or in some cases replace – fatality searches. Impact sensors mounted on turbine blades can capture the time a collision occurs, which may improve efforts to understand risk factors and to develop minimization strategies to reduce collision risk. Sensor readings would need to be paired with camera images or carcass searches to identify the species and any demographic data associated with the fatality.

Reporting PCM results

PCM results are typically reported as adjusted fatality rates per project and per megawatt (MW) of project nameplate capacity (maximum rated electricity output) per year. Raw counts from carcass searches must be adjusted to account for fatalities that occur outside the study period and carcasses that land outside the search area, as well as for two other sources of detection error: scavenger removal of carcasses and carcasses missed by searchers (searcher efficiency). Learn more about how raw carcass counts are adjusted to estimate actual fatality rates in Allison et al. 2019, p. 6.

How do we analyze these data?

PCM study reports from over 100 wind energy projects are publicly available, but many more PCM results are confidential and have been unavailable for analysis. The American Wind Wildlife Information Center (AWWIC) encourages voluntary data contributions from wind energy projects across the U.S. by maintaining data confidentiality, thus making more data available for analysis. AWWIC reports summarize bird and bat fatality rates (bird or bat fatalities per MW per year) and fatality incident data (individual fatalities) from wind energy facilities, including data from projects in regions with few published PCM studies.

AWWIC reports are updated regularly to provide the most up-to-date summary of fatality rates at hundreds of projects, and of species composition and timing of collisions to support the development of hypotheses that can be tested with additional analysis. It is important to note that these data do not represent a comprehensive or randomized monitoring dataset, and conclusions or extrapolations made from these data may change as additional data are added.

What do we know about collision fatalities of birds and bats?

For birds, 75% of studies in AWWIC reported 2.3 or fewer fatalities per MW per year, with a median fatality estimate of 1.3 birds per MW per year. (The median is reported instead of the mean because of the right-skewed distribution of fatality estimates.) Overall, bird fatality rates ranged from 0 to 19 fatalities per MW per year.

Adjusted bat fatality rates tend to be higher and more variable than bird fatality rates. 75% of studies in AWWIC reported estimates of fewer than 7.7 bat fatalities per MW per year, with a median of 3 bats per MW per year. Some projects along the forested ridgelines of the central Appalachians report rates close to 50 bats per MW per year.

Species at risk

  • The majority of bird fatalities are small passerines. Studies in AWWIC reported 307 species of birds discovered during PCM studies and an additional 13 that were found incidentally. Raw counts of small passerines (songbirds, which make up nearly 90% of all land birds) account for approximately 57% of fatalities reported in both publicly available and private studies conducted at U.S. wind facilities.
  • Fatalities of diurnal raptors are reported more often than expected given the relatively low abundance of these species. Diurnal raptors account for approximately 7% of reported fatalities. Red-tailed hawk and American kestrel are the most commonly reported fatalities; they are also the two most abundant diurnal raptors in the U.S. and have carcasses that tend to persist longer than other species and therefore may be more likely to be detected in searches.
  • Reported fatalities of other large bird species are very low. Collision fatalities of prairie grouse and waterbirds and waterfowl (such as ducks, gulls and terns, shorebirds, loons and grebes) are reported infrequently at land-based wind facilities.
  • The majority of bat fatalities are migratory tree-roosting bat species. At least 25 species of bats have been recorded as collision fatalities in North America, but approximately 70% of the reported fatalities at wind facilities for all North American regions combined are from three migratory tree-roosting species: hoary bat, eastern red bat, and silver-haired bat.
  • Mexican free-tailed bat, one of the most abundant bat species in the U.S., constitutes a substantial proportion (41–86%) of the estimated number of bats killed at wind facilities within this species’ range, which covers most of the southern half of the U.S.

Geographic variation

  • Bat fatality rates appear to vary substantially among regions in the U.S. Adjusted fatality rates of bats are highest at wind energy facilities in the upper Midwest and eastern forests and tend be much lower throughout the Great Plains and western U.S.
  • Raptor fatalities are higher in the Pacific region. The representation of diurnal raptors is much higher in the Pacific biome (21.7% of reported fatalities).

Seasonal patterns

  • Passerine fatalities are more common during migration seasons. Modest peaks in fatalities of small passerines occur during spring and fall at most wind facilities, presumably reflecting the passage of migrating species during these times.
    • Bat fatalities peak in the northern U.S. during late summer and early fall. Several studies in the northern U.S. have shown a peak in the incidence of bat fatalities in late summer and early fall, coinciding with the migration season of tree bats.

What else are we learning about collision risk factors?

Our level of certainty about risk factors reflects the weight of evidence, determined by the consistency of results across studies and the quality of the experimental designs employed. Learn more about how we define risk.

Conditions correlated with fatality patterns

  • Low wind speeds and warmer temperatures appear to be risk factors for bats. Bat activity is influenced by nightly wind speed and temperature, and some studies indicate that bat fatalities occur primarily on nights with low wind speeds. Other weather-related variables such as temperature, wind direction, or changing barometric pressure may also be important. Additional research on weather as a predictor of bat activity and fatalities could support targeted mitigation efforts to reduce bat fatalities.
  • Landscape and topography features may influence the abundance and risk-related behavior of large raptors around turbines. Large raptors are known to take advantage of wind currents created by ridge tops, upwind sides of slopes, and canyons that are favorable for local and migratory movements.
  • Broad-scale landscape features may be important predictors of fatality risk for migratory tree-bat species. In a study analyzing AWWIC bat fatality data from wind projects in the Midwest and Northeast, fatalities were higher at facilities that had a higher percentage of developed land within 25 km. Fatalities also appeared to be associated with wetland configuration, with increased risk in areas having multiple small or a combination of small and large wetland patches.

Potential risk factors that are not completely understood

  • It is unknown if collision risk increases with turbine height. The height and rotor-swept area of turbines has been increasing. It has been hypothesized that collision fatalities will also increase due to the greater overlap of taller turbines with flight heights of nocturnal-migrating songbirds and bats. Further, a larger rotor-swept area could increase the collision risk zone. A study is underway to explore the relationship between turbine height, rotor-swept area, and collision risk for both birds and bats.
  • Some bat species may be attracted to wind turbines. It has been hypothesized that the relatively high number of bat fatalities that have been observed for some species and locations may be explained by attraction to wind turbines or wind facilities. Several factors that might attract bats have been proposed, including the sounds produced by turbines, a concentration of insects near turbines, and bat mating behavior. However, definitive tests of these hypotheses are still needed.
  • The ability to predict collision risk for birds and bats from activity recorded by radar and acoustic detectors, respectively, remains elusive. The use of radar and bat acoustic detectors is a common feature of pre-construction risk assessments for siting wind energy facilities. To date, however, studies have not found a relationship between pre-construction activity surveys and post-construction collision risk. Predicting bat collision risk using pre-construction activity measures would be further complicated if bats are attracted to wind turbines.

Factors that do not appear to increase collision risk

  • Turbine lighting (FAA-recommended) does not increase collision risk to bats and migrating songbirds. The FAA regulates the lighting required on structures taller than 199 feet in height above ground level to ensure air traffic safety. The number of bat and songbird fatalities at turbines using FAA-approved lighting is not greater than that recorded at unlit turbines.
  • Barotrauma does not appear to be an important source of bat mortality at wind energy facilities. Forensic examination of bat carcasses found at wind energy facilities suggests that the importance of barotrauma, that is, injury resulting from rapidly altered air pressure caused by fast-moving wind turbine blades, is substantially less than originally suggested. A 2020 study comparing the pressure changes experienced by bats near a turbine blade with the pressure changes that cause barotrauma mortality in mammals similar in size to bats (rats and mice) concluded that it is unlikely that barotrauma is responsible for a significant number of turbine-related fatalities.

For more detailed information about collision impacts to wildlife from wind energy, view AWWI’s annually updated summary of wind turbine interactions with wildlife and their habitats.