Bats are dying in unprecedented numbers at wind turbines, but causes of their susceptibility are unknown. Fatalities peak during low-wind conditions in late summer and autumn and primarily involve species that evolved to roost in trees. Common behaviors of “tree bats” might put them at risk, yet the difficulty of observing high-flying nocturnal animals has limited our understanding of their behaviors around tall structures. We used thermal surveillance cameras for, to our knowledge, the first time to observe behaviors of bats at experimentally manipulated wind turbines over several months. We discovered previously undescribed patterns in the ways bats approach and interact with turbines, suggesting behaviors that evolved at tall trees might be the reason why many bats die at wind turbines.
Wind turbines are causing unprecedented numbers of bat fatalities. Many fatalities involve tree-roosting bats, but reasons for this higher susceptibility remain unknown. To better understand behaviors associated with risk, we monitored bats at three experimentally manipulated wind turbines in Indiana, United States, from July 29 to October 1, 2012, using thermal cameras and other methods. We observed bats on 993 occasions and saw many behaviors, including close approaches, flight loops and dives, hovering, and chases. Most bats altered course toward turbines during observation. Based on these new observations, we tested the hypotheses that wind speed and blade rotation speed influenced the way that bats interacted with turbines. We found that bats were detected more frequently at lower wind speeds and typically approached turbines on the leeward (downwind) side. The proportion of leeward approaches increased with wind speed when blades were prevented from turning, yet decreased when blades could turn. Bats were observed more frequently at turbines on moonlit nights. Taken together, these observations suggest that bats may orient toward turbines by sensing air currents and using vision, and that air turbulence caused by fast-moving blades creates conditions that are less attractive to bats passing in close proximity. Tree bats may respond to streams of air flowing downwind from trees at night while searching for roosts, conspecifics, and nocturnal insect prey that could accumulate in such flows. Fatalities of tree bats at turbines may be the consequence of behaviors that evolved to provide selective advantages when elicited by tall trees, but are now maladaptive when elicited by wind turbines.
Paul. M. Cryan
Fort Collins Science Center, United States Geological Survey (USGS), Fort Collins, CO
P. Marcos Gorresen
Hawaii Cooperative Studies Unit, University of Hawaii, Hilo, HI
Cris D. Hein
Michael R. Schirmacher
Bat Conservation International, Austin, TX
Robert H. Diehl
Northern Rocky Mountain Science Center, USGS, Bozeman, MT
Manuela M. Huso
Forest and Rangeland Ecosystem Science Center, USGS, Corvallis, OR
David T. S. Hayman
Department of Biology, Colorado State University, Fort Collins, CO
Department of Biology, University of Florida, Gainesville, FL
Paul D. Fricker
MathWorks, Natick, MA
Frank J. Bonaccorso
Pacific Island Ecosystems Research Center, USGS, Hawaii National Park, HI
Douglas H. Johnson
Northern Prairie Wildlife Research Center, USGS, Saint Paul, MN
Conservation Biology Graduate Program, University of Minnesota, Saint Paul, MN 55108
David C. Dalton
Wildlife Engineering, Tucson, AZ 85745
PNAS (Proceedings of the National Academy of Sciences of the United States of America) 2014; published ahead of print September 29, 2014; doi: 10.1073/pnas.1406672111 
Download original document: “Behavior of bats at wind turbines ”
Download “Supporting Information” 
Movie S2. Bat making repeated looping approaches to leeward side of wind turbine at ∼0109 hours on August 29, 2012. Blade rotation <1 rpm, wind out of the east-northeast (58°) at 5.4 m/s, and 93% moon illumination.
Movie S3. A hoary bat (Lasiurus cinereus; identified acoustically) air-brakes, hovers, and then makes repeated approaches after flying downwind past a wind turbine with curtailed blades at ∼0100 hours on August 25, 2012. Blade rotation <1 rpm, wind out of the southeast (131°) at 7.2 m/s, and no moon illumination.
Movie S4. Near-infrared, close-up video of a bat closely approaching and investigating the upper parts of a turbine at ∼0430 hours on September 19, 2013. Blade rotation <1 rpm, wind out of the west-southwest (257°) at 2.7 m/s, and no moon illumination.
Movie S5. Bat making repeated close approaches to a turbine monopole at ∼2150 hours on August 19, 2012. No blade rotation, wind out of the north-northwest (330°) at 0.4 m/s, and no moon illumination.
Movie S6. Near-infrared, close-up video of a bat closely following a slow moving turbine blade (shadowed on far side of monopole) at ∼0240 hours on July 19, 2013 (before monitoring with thermal cameras began). Blade rotation <1 rpm, wind out of the east-northeast (70°) at 7.5 m/s, and no moon illumination.
Movie S7. Two bats chasing each other near wind turbine at ∼2320 hours on August 5, 2012. No blade rotation, wind out of the north-northwest (321°) at 4.6 m/s, and no moon illumination.
Movie S8. Bat repeatedly returning to turbine after close encounters with spinning blades at ∼0150 hours on August 22, 2012. Blade rotation 14 rpm, wind out of the east (93°) at 8.0 m/s, and no moon illumination.
Movie S9. Bat exhibiting serpentine flight in lee of wind turbine monopole and blades at ∼0500 hours on September 29, 2012. No blade rotation, wind out of the northeast (315°) at 5.8 m/s, and 96% moon illumination.
Movie S10. Two hoary bats (Lasiurus cinereus) interacting in midair on the leeward side of a wind turbine at ∼0200 hours on August 25, 2012. The species identification was made from concurrent acoustic calls recorded from the turbine nacelle, in which navigation and social calls characteristic of this species were heard during the close midair approaches. No blade rotation, wind out of the south-southeast (157°) at 8.3 m/s, and no moon illumination.
Movie S11. Bat flying upwind to investigate leeward areas of a wind turbine with blades rotating at full speed at ∼0350 hours on July 31, 2012. Bat makes several upwind passes through the moving blades of the turbine without clear indication that it perceives and avoids the fast-moving blades before moving through their plane of motion. Blade rotation speed 14 rpm, wind out of the southwest (228°) at 7.2 m/s, and 95% moon illumination.
Movie S12. Bat flying upwind toward moving turbine blades at ∼0600 hours on August 17, 2012 and repeatedly returning to investigate after close encounters with blades. Blade rotation speed 14 rpm, wind out of the north-northwest (324°) at 7.6 m/s, and no moon illumination.
URL to article: https://www.wind-watch.org/documents/behavior-of-bats-at-wind-turbines/
URLs in this post:
 doi: 10.1073/pnas.1406672111: http://dx.doi.org/10.1073/pnas.1406672111
 Behavior of bats at wind turbines: https://docs.wind-watch.org/Cryan-bats-PNAS-2014.pdf
 Download “Supporting Information”: https://docs.wind-watch.org/Cryan-bats-PNAS-2014-SI.pdf