title: Foraging at the edge of the world: low-altitude, high-speed manoeuvering in barn swallows.
abstract: While prior studies of swallow manoeuvering have focused on slow-speed flight and obstacle avoidance in still air, swallows survive by foraging at high speeds in windy environments. Recent advances in field-portable, high-speed video systems, coupled with precise anemometry, permit measures of high-speed aerial performance of birds in a natural state. We undertook the present study to test: (i) the manner in which barn swallows (Hirundo rustica) may exploit wind dynamics and ground effect while foraging and (ii) the relative importance of flapping versus gliding for accomplishing high-speed manoeuvers. Using multi-camera videography synchronized with wind-velocity measurements, we tracked coursing manoeuvers in pursuit of prey. Wind speed averaged 1.3-2.0 m s(-1) across the atmospheric boundary layer, exhibiting a shear gradient greater than expected, with instantaneous speeds of 0.02-6.1 m s(-1) While barn swallows tended to flap throughout turns, they exhibited reduced wingbeat frequency, relying on glides and partial bounds during maximal manoeuvers. Further, the birds capitalized on the near-earth wind speed gradient to gain kinetic and potential energy during both flapping and gliding turns; providing evidence that such behaviour is not limited to large, fixed-wing soaring seabirds and that exploitation of wind gradients by small aerial insectivores may be a significant aspect of their aeroecology.This article is part of the themed issue 'Moving in a moving medium: new perspectives on flight'.
authors.affiliation: Department of Integrative Biology, Oregon State University, Corvallis, OR 97331, USA warrickd@science.oregonstate.edu.
authors.forename: Douglas R
authors.initials: DR
authors.lastname: Warrick
authors.affiliation: Department of Biology, University of North Carolina at Chapel Hill, NC 27599, USA.
authors.forename: Tyson L
authors.initials: TL
authors.lastname: Hedrick
authors.affiliation: Concord Field Station, Department of Organismic and Evolutionary Biology, Harvard University, Old Causeway Road, Bedford, MA 01730, USA.
authors.forename: Andrew A
authors.identifier: 0000-0003-3303-8737
authors.initials: AA
authors.lastname: Biewener
authors.affiliation: Field Research Station at Fort Missoula, Division of Biological Sciences, University of Montana, Missoula, MT 59812, USA Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK.
authors.forename: Kristen E
authors.initials: KE
authors.lastname: Crandell
authors.affiliation: Field Research Station at Fort Missoula, Division of Biological Sciences, University of Montana, Missoula, MT 59812, USA.
authors.forename: Bret W
authors.identifier: 0000-0002-5739-6099
authors.initials: BW
authors.lastname: Tobalske
mesh_terms: D000818:Animals; D005247:Feeding Behavior; D005260:Female; D005426:Flight, Animal; D008297:Male; D009922:Oregon; D046388:Swallows / Q000502:physiology*; D014919:Wind
keywords: flapping; foraging flight; gliding; kinematics; wind energy