title: Hovering performance of Anna's hummingbirds (Calypte anna) in ground effect.
abstract: Aerodynamic performance and energetic savings for flight in ground effect are theoretically maximized during hovering, but have never been directly measured for flying animals. We evaluated flight kinematics, metabolic rates and induced flow velocities for Anna's hummingbirds hovering at heights (relative to wing length R = 5.5 cm) of 0.7R, 0.9R, 1.1R, 1.7R, 2.2R and 8R above a solid surface. Flight at heights less than or equal to 1.1R resulted in significant reductions in the body angle, tail angle, anatomical stroke plane angle, wake-induced velocity, and mechanical and metabolic power expenditures when compared with flight at the control height of 8R. By contrast, stroke plane angle relative to horizontal, wingbeat amplitude and wingbeat frequency were unexpectedly independent of height from ground. Qualitative smoke visualizations suggest that each wing generates a vortex ring during both down- and upstroke. These rings expand upon reaching the ground and present a complex turbulent interaction below the bird's body. Nonetheless, hovering near surfaces results in substantial energetic benefits for hummingbirds, and by inference for all volant taxa that either feed at flowers or otherwise fly close to plant or other surfaces.
authors.affiliation: Biophysics Graduate Program, University of California, Berkeley, CA 94720-3200, USA.
authors.forename: Erica J
authors.initials: EJ
authors.lastname: Kim
authors.affiliation: Department of Biology, Lund University, Lund, Sweden.
authors.forename: Marta
authors.initials: M
authors.lastname: Wolf
authors.affiliation: Department of Integrative Biology, University of California, Berkeley, CA 94720-3200, USA vortega@berkeley.edu.
authors.forename: Victor Manuel
authors.initials: VM
authors.lastname: Ortega-Jimenez
authors.affiliation: Department of Molecular Cell Biology, University of California, Berkeley, CA 94720-3200, USA.
authors.forename: Stanley H
authors.initials: SH
authors.lastname: Cheng
authors.affiliation: Department of Integrative Biology, University of California, Berkeley, CA 94720-3200, USA Smithsonian Tropical Research Institute, Balboa, Republic of Panama.
authors.forename: Robert
authors.initials: R
authors.lastname: Dudley
mesh_terms: D000392:Air Movements; D000818:Animals; D001696:Biomechanical Phenomena; D001717:Birds / Q000502:physiology*; D004734:Energy Metabolism; D005426:Flight, Animal; D008297:Male; D008962:Models, Theoretical; D014921:Wings, Animal / Q000502:physiology
keywords: ground effect; hovering; induced velocity; metabolic power; vortex wake