Gliding Flight of the Andean Condor in Nature

ABSTRACT Derived in a vector analysis with measurements of wind velocity and ground velocity of the bird, the following mean air speeds were obtained for birds crossing a Peruvian beach: 15 m/sec for 15 gliding Andean condors, 14 m/sec for 42 condors that flapped during the crossing, and 10 m/sec for five turkey vultures that flapped. For the 15 gliding condors a mean lift coefficient of 0 · 7 and a mean induced drag force of 3 N were computed. Implausibly low values derived for parasite drag coefficient of the condor appeared to be due to (a) unmeasured forces of deceleration and (b) an undetected vertical component of the wind at the level of the flight path. Field data, adjusted by introducing a coefficient of parasite drag determined for the black vulture in a windtunnel study provided corrected estimates of drag. I secured an adjusted value of 14 for the L/D ratio of a condor gliding with wings fully extended. A moderate flexion of the wings reducing the span by 20% is estimated to increase the optimum air speed from 13 · 9 to 15 · 2 m/sec for an adult male condor and from 12 · 6 to 13 · 8 m/sec for an adult female.