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In gliding flight, birds morph their wings and tails to control their flight trajectory and speed. Using high-resolution videogrammetry, we reconstructed accurate and detailed three-dimensional geometries of gliding flights for three raptors (barn owl, Tyto alba; tawny owl, Strix aluco, and goshawk, Accipiter gentilis). Wing shapes were highly repeatable and shoulder actuation was a key component of reconfiguring the overall planform and controlling angle of attack. The three birds shared common spanwise patterns of wing twist, an inverse relationship between twist and peak camber, and held their wings depressed below their shoulder in an anhedral configuration. With increased speed, all three birds tended to reduce camber throughout the wing, and their wings bent in a saddle-shape pattern. A number of morphing features suggest that the coordinated movements of the wing and tail support efficient flight, and that the tail may act to modulate wing camber through indirect aeroelastic control.
Bibliographical noteFunding Information:
Ethics. The experimental work complied with all relevant ethical regulations and was approved by the Ethics and Welfare Committee of the Royal Veterinary College (URN 2015 1358) and the University of Bristol Animal Welfare and Ethical Review Body (UIN UB/15/070). Data accessibility. The data are provided in electronic supplementary material . Authors’ contributions. J.A.C., J.P.J.S., N.E.D., S.P.W., J.R.U. and R.J.B. conceived and designed the experiment. J.A.C., J.P.J.S., N.E.D. and R.J.B. set up the experiment. J.A.C., J.P.J.S., N.E.D., S.P.W., J.R.U. and R.J.B. ran the experiment. D.A.M.-S. facilitated photogrammetric processing on the Bluecrystal HPC. J.A.C., J.P.J.S. and N.E.D. performed photogrammetric reconstruction of the birds. J.A.C., M.M. and J.S. built the three-dimensional geometries. J.A.C., J.P.J.S., N.E.D., M.M., J.S., S.P.W., J.R.U. and R.J.B. analysed the results. All authors contributed to writing the manuscript. Competing interests. We declare we have no competing interests. Funding. This work was supported by the Air Force Office of Scientific Research, Air Force Materiel Command, USAF under award no. FA9550-16-1-0034; and by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 679355). J.R.U. was funded by a Wellcome Trust Fellowship 202854/Z/16/Z. M.M. was supported by BBSRC grant no. BB/R002657/1 to R.J.B. Acknowledgements. We would like to thank Kris Crandell for feedback on the manuscript. We also acknowledge the helpful and supportive roles of Nathan Phillips, Maja Lorenc and Tim West in collecting these measurements. We thank Shintaro Ichihara, Mizue Inumaru, DVM, Ryota Shinya and Masanori Tatani for verifying feather thickness measurements of the A. gentilis geometry. Finally, we thank Lloyd and Rose Buck for their assistance with, and training of, the raptors examined in this study. This work was carried out using the computational facilities of the Advanced Computing Research Centre, University of Bristol, Bristol, UK.
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