Airspeed adjustment and compensation for wind drift in the common swift
Abstract: Migratory birds are known to be capable of adjusting their heading direction to compensate for wind drift and their airspeed adaptively with respect to head and tail winds. High-flying nocturnally migrating common swifts, Apus apus, have been shown to compensate for wind drift, but they failed to adjust airspeed as expected (increase in head wind and decrease in tail wind in relation to neutral wind). We report on new measurements of diurnally migrating common swifts at a coastal site in the Baltic, where the birds did adjust airspeed adaptively during spring and autumn migration. During autumn migration, they compensated for lateral wind drift by adjusting heading direction similarly to high-altitude migrants in autumn. We also recorded flight speed and wind compensation during a summer weather-related exodus, when the birds behaved similarly to those during autumn migration, although they showed a small degree of wind drift. Why birds failed to adjust airspeed adaptively at high altitude is discussed, and we argue there is a threshold in the sensory system to detect small changes in optic flow based on visual landmarks.