Abstract
This report provides the most up-to-date snapshot of the patterns and variability in species composition, timing, and magnitude of bat collisions with land-based wind energy turbines. Findings are useful in checking assumptions and setting expectations about collision risks at wind energy facilities, as well as generating testable hypotheses and assessing monitoring designs.
Background
This report summarizes data from 338 postconstruction mortality monitoring studies conducted over 22 years and across 256 land-based wind energy projects in the United States.
- The AWWIC database contains fatality estimates and protocols used to develop those estimates, individual fatality incident records, and information about the wind energy project itself (such as turbine size, installed capacity, ecoregion).
- Data are summarized by U.S. Fish and Wildlife Service regions and states that contain data from at least five different wind projects.
Carcass detection
- Approximately 90% of bat carcasses were found within 50m of the turbine, and nearly 100% within 100m.
- Full plot searches provided much greater density weighted proportion (DWP): median 0.91 vs. 0.15 for road & pad searches.
- Estimated carcass persistence times (median 4.2 days) and search efficiency (median 64%) were similar across regions.
Bat fatalities
- Reported bat fatality incidents were widespread, but infrequent: Nearly all (96.4%) studies report at least 1 bat fatality, yet 96.9% of turbine-searches found zero carcasses.
- Estimated bat fatality rates differed greatly among studies: Many had relatively low fatality estimates and few had higher values; fatality estimates tended to be greater in the Midwest.
- The median fatality estimate for all bats in the U.S. was 3.1 bats per MW per year and 6.2 bats per turbine per year
- Studies with estimated fatalities >5 bats per MW per year were concentrated in the Midwest (median 8.7 bats per MW per year).
- Widespread species of migratory tree bats (hoary bat, eastern red bat, and silver-haired bat) contributed >70% of fatality incidents.
- U.S. federally listed bat species (Indiana bat and northern long-eared myotis) contributed <10 total fatalities in AWWIC
- Tricolored bat, currently awaiting a listing decision under the ESA, accounted for ~2% of fatality records.
- Bat fatalities peaked August — September, across species and regions.
- Peak occurred earlier in more northerly regions.
- Silver-haired bats had a much smaller secondary uptick in fatalities in spring (AprilJune), as shown by Lloyd et al. (2023).
Since the first edition of this report in 2018:
Additional data from 126 wind energy projects have led to the following broad insights on bat collisions at wind energy facilities:
- Total number of bat species discovered as fatalities at wind energy facilities has remained the same (22)
- Overall species composition of migratory tree bats has remained 72%
- Late summer through early fall has continued to have the highest percentage of fatality incidents in all species and regions, although the peak fatality timing varies by species and region.
- The number of wind turbines with exceptional numbers of bat carcass finds has remained low. Turbines with 10 or more carcasses found during an entire study dropped from 8% in the first edition to 2.8% in the third edition.
- The all-species median bat fatality rate has increased from 2.7 to 3.1 bats per MW per year. However, this increase is largely due to the Midwest Region increase from 6.2 to 8.7 as little or no increase occurred in other regions.