Twilight ascents by common swifts, Apus apus, at dawn and dusk: acquisition of orientation cues?

► Flight altitudes of common swifts were monitored full-season by weather radar. ► Common swifts undertake twilight ascents both at dusk and dawn. ► Twilight ascents show a temporal mirror symmetry referenced to sunset/sunrise. ► Ascent height is positively correlated with atmospheric temperature. ► Twilight ascents are probably associated with orientation challenges.