Turbulence explains the accelerations of an eagle in natural flight

Turbulent winds and gusts fluctuate on a wide range of timescales from milliseconds to minutes and longer, a range that overlaps the timescales of avian flight behavior, yet the importance of turbulence to avian behavior is unclear. By combining wind speed data with the measured accelerations of a g...

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Published inProceedings of the National Academy of Sciences - PNAS Vol. 118; no. 23; pp. 1 - 6
Main Authors Laurent, Kasey M., Fogg, Bob, Ginsburg, Tobias, Halverson, Casey, Lanzone, Michael J., Miller, Tricia A., Winkler, David W., Bewley, Gregory P.
Format Journal Article
LanguageEnglish
Published Washington National Academy of Sciences 08.06.2021
SeriesFrom the Cover
Subjects
Aerodynamics
Atmospheric turbulence
Biological Sciences
Birds
Convection
Convection cells
Flight
Flight behavior
Gusts
Physical Sciences
Power spectra
Turbulence intensity
Turbulent wind
Wildlife
Wind speed
Wing span
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Summary:Turbulent winds and gusts fluctuate on a wide range of timescales from milliseconds to minutes and longer, a range that overlaps the timescales of avian flight behavior, yet the importance of turbulence to avian behavior is unclear. By combining wind speed data with the measured accelerations of a golden eagle (Aquila chrysaetos) flying in the wild, we find evidence in favor of a linear relationship between the eagle’s accelerations and atmospheric turbulence for timescales between about 1/2 and 10 s. These timescales are comparable to those of typical eagle behaviors, corresponding to between about 1 and 25 wingbeats, and to those of turbulent gusts both larger than the eagle’s wingspan and smaller than large-scale atmospheric phenomena such as convection cells. The eagle’s accelerations exhibit power spectra and intermittent activity characteristic of turbulence and increase in proportion to the turbulence intensity. Intermittency results in accelerations that are occasionally several times stronger than gravity, which the eagle works against to stay aloft. These imprints of turbulence on the bird’s movements need to be further explored to understand the energetics of birds and other volant life-forms, to improve our own methods of flying through ceaselessly turbulent environments, and to engage airborne wildlife as distributed probes of the changing conditions in the atmosphere.
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Author contributions: D.W.W. and G.P.B. designed research; B.F., C.H., and M.J.L. performed research and provided data; K.M.L., T.G., and D.W.W. analyzed data; K.M.L., T.A.M., and G.P.B. wrote the paper; and T.A.M. and D.W.W. revised the manuscript critically for important intellectual content.
Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved May 3, 2021 (received for review February 12, 2021)
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.2102588118