The gliding speed of migrating birds: slow and safe or fast and risky?

Authors

  • Nir Horvitz,

    Corresponding author
    1. Movement Ecology Laboratory, Department of Ecology, Evolution and Behavior, Alexander Silberman Institute of Life Sciences, Edmond J. Safra campus, The Hebrew University of Jerusalem, Jerusalem, Israel
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    • These authors contributed equally to this work.
  • Nir Sapir,

    1. Movement Ecology Laboratory, Department of Ecology, Evolution and Behavior, Alexander Silberman Institute of Life Sciences, Edmond J. Safra campus, The Hebrew University of Jerusalem, Jerusalem, Israel
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    • These authors contributed equally to this work.
  • Felix Liechti,

    1. Swiss Ornithological Institute, Sempach, Switzerland
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  • Roni Avissar,

    1. Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA
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  • Isaac Mahrer,

    1. The Seagram Center for Soil and Water Sciences, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel
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  • Ran Nathan

    1. Movement Ecology Laboratory, Department of Ecology, Evolution and Behavior, Alexander Silberman Institute of Life Sciences, Edmond J. Safra campus, The Hebrew University of Jerusalem, Jerusalem, Israel
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Abstract

Aerodynamic theory postulates that gliding airspeed, a major flight performance component for soaring avian migrants, scales with bird size and wing morphology. We tested this prediction, and the role of gliding altitude and soaring conditions, using atmospheric simulations and radar tracks of 1346 birds from 12 species. Gliding airspeed did not scale with bird size and wing morphology, and unexpectedly converged to a narrow range. To explain this discrepancy, we propose that soaring-gliding birds adjust their gliding airspeed according to the risk of grounding or switching to costly flapping flight. Introducing the Risk Aversion Flight Index (RAFI, the ratio of actual to theoretical risk-averse gliding airspeed), we found that inter- and intraspecific variation in RAFI positively correlated with wing loading, and negatively correlated with convective thermal conditions and gliding altitude, respectively. We propose that risk-sensitive behaviour modulates the evolution (morphology) and ecology (response to environmental conditions) of bird soaring flight.

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