Testing an Emerging Paradigm in Migration Ecology Shows Surprising Differences in Efficiency between Flight Modes

et al. (2012) Testing an Emerging Paradigm in Migration Ecology Shows Surprising Differences in Efficiency between Flight Modes. PLoS ONE 7(4): e35548. doi:10.1371/journal.pone.0035548 Testing an Emerging Paradigm in Migration Ecology Shows Surprising Differences in Efficiency between Flight Modes Adam E. Duerr 0 Tricia A. Miller 0 Michael Lanzone 0 Dave Brandes 0 Jeff Cooper 0 Kieran O'Malley 0 Charles Maisonneuve 0 Junior Tremblay 0 Todd Katzner 0 Yan Ropert-Coudert, Institut Pluridisciplinaire Hubert Curien, France 0 1 Division of Forestry and Natural Resources, West Virginia University , Morgantown , West Virginia, United States of America , 2 Riparia , The Pennsylvania State University, University Park, Pennsylvania, United States of America, 3 Cellular Tracking Technologies LLC, Somerset, Pennsylvania, United States of America, 4 Department of Civil and Environmental Engineering, Acopian Engineering Center, Lafayette College, Easton, Pennsylvania, United States of America, 5 Virginia Department of Game and Inland Fisheries , Fredericksburg , Virginia, United States of America, 6 West Virginia Division of Natural Resources, Romney, West Virginia, United States of America, 7 Ministe`re des Ressources naturelles et de la Faune , Rimouski, Que bec , Canada , 8 Ministe`re des Ressources naturelles et de la Faune , Que bec City, Que bec , Canada , 9 United States Department of Agriculture, Forest Service, Timber and Watershed Laboratory , Parsons, West Virginia , United States of America 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. - Funding: Funding was received from Pennsylvania State Wildlife Grants T-12 and T47-R-1, United States Department of Energy grant DE-EE0003538, the Virginia Department of Game and Inland Fisheries, Invenergy LLC, and the West Virginia Division of Natural Resources. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript that are not represented through coauthorship of this manuscript. Competing Interests: TK and ML are also owners or Cellular Tracking Technologies, LLC that manufacture GPS-GSM transmitters used in this project. Funding was received from Invenergy LLC, a wind power generation company. Funding was also received from two state agencies that employ coauthors JC and KO. This does not alter the authors adherence to all the PLoS ONE policies on sharing data and materials. Movement has dramatic consequences for demography and thus fitness [1]. Animals that undertake long-distance movements face trade-offs between minimizing time and minimizing energetic expenditures [2,3]. Choosing incorrectly in these movements can have dramatic selective consequences [4,5,6]. Migration by birds progresses primarily through combinations of two flight types: straight-winged flight modes (soaring or gliding) and flapping flight. Knowing absolute and relative speeds of different flight types and modes is important to understand how energetically-or time-constrained animals move. Understanding flight speeds is also crucial to evaluating the influence of flight modes on the evolution of migration routes and wing morphology, and the complex trade-offs between time and energy when migrating [2,3]. However, in spite of the importance of evaluating these processes, most studies that measure instantaneous or average flight speeds do not distinguish between different modes of flight [7,8,9,10]. This is likely because comparison of speeds of different modes of soaring has been technologically difficult or impossible to achieve, even for large birds (e.g., [11,12]). Flight strategies used by large birds differ from those of small birds due to relationships between energetic costs of flight and bird mass. Although body shape, wing shape, and wing loading also affect flight energetics [11], energetic demands of flapping flight generally increase geometrically with body mass (E/M1.17; [13]; Fig. 1). Thus, for birds with high body mass, energetic costs during flapping flight can be several times their basal metabolic rate (BMR). In contrast, energy required for soaring and gliding are proportionally lower, around twice that of BMR [14,15]. Furthermore, BMR increases with body mass (BMR/M0.78; [16]) at a much slower rate than energetic requirements of flapping flight; therefore, soaring becomes an increasingly efficient mode of flight as mass increases [17,18]. Field observations support flight theory. Heavier species of harrier (Circus spp.) soared more and used flapping flight less than lighter harriers during migration [19]. The costs of flapping flight are dramatically apparent in observations of short-toed eagles (Circaetus gallicus) that extend migration routes 5001700 km to avoid flapping flight over water [20] and griffon vultures (Gyps fulvus), among the heaviest flighted birds, which died at narrow sea crossings when forced only to use flapping flight [6]. Soaring is the use of air currents to aid in straight-winged flight with the two most prevalent modes over land being thermal and slope soaring [17]. First, thermal soaring is use of heated rising air to gain altitude. Differential heating of the earth causes surface layers of air to warm and rise, forming updrafts that can extend into lower layers of the atmosphere. Thermals develop during relatively calm conditions but break down with strong winds [21,22]. Birds gain altitude by circling in these rising air currents during thermal soaring and they glide between them to make forwar (...truncated)