Virtual manipulation of bird tail postures demonstrates drag minimisation when gliding

Aerodynamic function of the avian tail has previously been studied using observations of bird flight, physical models in wind tunnels, theoretical modelling and flow visualization. However, none of these approaches has provided rigorously quantitative evidence concerning tail functions since: appropriate manipulation and controls cannot be achieved using live animals; and the aerodynamic interplay with wings and body challenges reductive theoretical or physical modelling approaches. Here, we have developed a comprehensive analytical drag model, calibrated by high-fidelity computational fluid dynamics (CFD), and used it to investigate the aerodynamic action of the tail by virtually manipulating its posture. The bird geometry used for CFD was reconstructed previously using stereo photogrammetry of a freely gliding barn owl and validated against wake measurements. Using this CFD-calibrated drag model, we predicted the drag production for 16 gliding flights with a range of tail postures. These observed postures are set in the context of a wider parameter sweep of theoretical postures, where the tail spread and elevation angles were manipulated virtually. The observed postures of our gliding bird corresponded to near minimal total drag.