Wing movements and positioning for aerodynamic benefit by Canada geese flying in formation

Abstract

Wing tip spacing (the distance between wing tips of adjacent birds at maximum span, perpendicular to the flight path), depth (distance between adjacent birds along the flight path), wing beat frequencies, and extreme relative wing positions were measured for Canada geese (Branta canadensis) flying in V formations to test for use of variation in trailing wing tip vortex positions produced by wing movements. Use of vertical vortex position variation requires similarity in wing beat frequency. An average of only 48% of 73 birds in eight formations had frequencies similar to those of the bird ahead during migratory flight (difference ≤ 0.1 beat/s). Birds whose wing beat frequency was similar to that of the bird ahead differed in depth based on whether wings were in or out of phase. Use of horizontal vortex position variation involves variation in wing tip spacing with depth, which was observed, but variation was high and median wing tip spacing was less for birds with similar wing beat frequencies to the bird ahead in only two of eight formations. Induced power saving may be limited by unpredictable moves of birds ahead and by ability to track trailing vortex positions.