title: Testing an emerging paradigm in migration ecology shows surprising differences in efficiency between flight modes.
abstract: To maximize fitness, flying animals should maximize flight speed while minimizing energetic expenditure. Soaring speeds of large-bodied birds are determined by flight routes and tradeoffs between minimizing time and energetic costs. Large raptors migrating in eastern North America predominantly glide between thermals that provide lift or soar along slopes or ridgelines using orographic lift (slope soaring). It is usually assumed that slope soaring is faster than thermal gliding because forward progress is constant compared to interrupted progress when birds pause to regain altitude in thermals. We tested this slope-soaring hypothesis using high-frequency GPS-GSM telemetry devices to track golden eagles during northbound migration. In contrast to expectations, flight speed was slower when slope soaring and eagles also were diverted from their migratory path, incurring possible energetic costs and reducing speed of progress towards a migratory endpoint. When gliding between thermals, eagles stayed on track and fast gliding speeds compensated for lack of progress during thermal soaring. When thermals were not available, eagles minimized migration time, not energy, by choosing energetically expensive slope soaring instead of waiting for thermals to develop. Sites suited to slope soaring include ridges preferred for wind-energy generation, thus avian risk of collision with wind turbines is associated with evolutionary trade-offs required to maximize fitness of time-minimizing migratory raptors.
authors.affiliation: Division of Forestry and Natural Resources, West Virginia University, Morgantown, West Virginia, United States of America. adam.duerr@mail.wvu.edu
authors.forename: Adam E
authors.initials: AE
authors.lastname: Duerr
authors.forename: Tricia A
authors.initials: TA
authors.lastname: Miller
authors.forename: Michael
authors.initials: M
authors.lastname: Lanzone
authors.forename: Dave
authors.initials: D
authors.lastname: Brandes
authors.forename: Jeff
authors.initials: J
authors.lastname: Cooper
authors.forename: Kieran
authors.initials: K
authors.lastname: O'Malley
authors.forename: Charles
authors.initials: C
authors.lastname: Maisonneuve
authors.forename: Junior
authors.initials: J
authors.lastname: Tremblay
authors.forename: Todd
authors.initials: T
authors.lastname: Katzner
mesh_terms: D000531:Altitude; D025041:Animal Migration / Q000502:physiology*; D000818:Animals; D001696:Biomechanical Phenomena; D020307:Eagles / Q000502:physiology*; D004463:Ecology; D004734:Energy Metabolism / Q000502:physiology*; D005426:Flight, Animal / Q000502:physiology*; D008297:Male; D013686:Telemetry; D014919:Wind; D014921:Wings, Animal / Q000502:physiology