Optimal use of wind by migrating birds: combined drift and overcompensation

Migrating birds may save flying time by allowing themselves to be partially drifted by strong winds at high altitude and correcting for the displacement at low altitude under relatively weaker winds. This behaviour will be favourable with strong upper winds and with wind direction approx. 30 ° to 90 ° in relation to the goal direction (following side winds). Radar observations of drift in high altitude bird migration and visual records of low altitude overcompensation are compatible with the optimal flight behaviour of migrants at high and low altitude, respectively, as predicted from this hypothesis.