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

Nir Horvitz, Nir Sapir, Felix Liechti, Roni Avissar, Isaac Mahrer, Ran Nathan

Research output: Contribution to journalArticlepeer-review

53 Scopus citations


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.

Original languageEnglish
Pages (from-to)670-679
Number of pages10
JournalEcology Letters
Issue number6
StatePublished - 1 Jan 2014
Externally publishedYes


  • Atmospheric modelling
  • Bird migration
  • Convective thermals
  • Flight behaviour
  • Movement ecology
  • Risk-sensitive flight
  • Soaring flight
  • Tracking radar
  • Turbulence kinetic energy
  • Wing loading

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics


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