title: The gliding speed of migrating birds: slow and safe or fast and risky?
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.
authors.affiliation: Movement Ecology Laboratory, Department of Ecology, Evolution and Behavior, Alexander Silberman Institute of Life Sciences, Edmond J. Safra campus, The Hebrew University of Jerusalem, 91904, Jerusalem, Israel.
authors.forename: Nir
authors.initials: N
authors.lastname: Horvitz
authors.forename: Nir
authors.initials: N
authors.lastname: Sapir
authors.forename: Felix
authors.initials: F
authors.lastname: Liechti
authors.forename: Roni
authors.initials: R
authors.lastname: Avissar
authors.forename: Isaac
authors.initials: I
authors.lastname: Mahrer
authors.forename: Ran
authors.initials: R
authors.lastname: Nathan
mesh_terms: D025041:Animal Migration; D000818:Animals; D001522:Behavior, Animal / Q000502:physiology; D005075:Biological Evolution; D001696:Biomechanical Phenomena; D001717:Birds / Q000033:anatomy & histology* / Q000502:physiology*; D005426:Flight, Animal / Q000502:physiology*; D013045:Species Specificity; D014921:Wings, Animal / Q000033:anatomy & histology
keywords: Atmospheric modelling; bird migration; convective thermals; flight behaviour; movement ecology; risk-sensitive flight; soaring flight; tracking radar; turbulence kinetic energy; wing loading