title: Structure of the vortex wake in hovering Anna's hummingbirds (Calypte anna).
abstract: Hummingbirds are specialized hoverers for which the vortex wake has been described as a series of single vortex rings shed primarily during the downstroke. Recent findings in bats and birds, as well as in a recent study on Anna's hummingbirds, suggest that each wing may shed a discrete vortex ring, yielding a bilaterally paired wake. Here, we describe the presence of two discrete rings in the wake of hovering Anna's hummingbirds, and also infer force production through a wingbeat with contributions to weight support. Using flow visualization, we found separate vortices at the tip and root of each wing, with 15% stronger circulation at the wingtip than at the root during the downstroke. The upstroke wake is more complex, with near-continuous shedding of vorticity, and circulation of approximately equal magnitude at tip and root. Force estimates suggest that the downstroke contributes 66% of required weight support, whereas the upstroke generates 35%. We also identified a secondary vortex structure yielding 8-26% of weight support. Lift production in Anna's hummingbirds is more evenly distributed between the stroke phases than previously estimated for Rufous hummingbirds, in accordance with the generally symmetric down- and upstrokes that characterize hovering in these birds.
authors.affiliation: Department of Integrative Biology, University of California, , Berkeley, CA 94720, USA, Department of Biology, Lund University, , Lund 223 62, Sweden, Smithsonian Tropical Research Institute, , Balboa, Republic of Panama.
authors.forename: M
authors.initials: M
authors.lastname: Wolf
authors.forename: V M
authors.initials: VM
authors.lastname: Ortega-Jimenez
authors.forename: R
authors.initials: R
authors.lastname: Dudley
mesh_terms: D000392:Air Movements; D000818:Animals; D001696:Biomechanical Phenomena; D001717:Birds / Q000033:anatomy & histology / Q000502:physiology*; D005426:Flight, Animal; D014921:Wings, Animal / Q000033:anatomy & histology / Q000502:physiology
keywords: aerodynamics; flight; hovering; hummingbird; lift; vortex wake