Flight behaviour of Common Swifts

By making use of long-term weather radar observations in the Netherlands, we discovered that common swifts (Apus apus) make ascents to high altitudes (up to 3 km) both at dawn and dusk. The dusk ascent of swifts was previously considered to be related to the onset of nocturnal roosting, which is performed on the wing. The discovery that swifts also climb to high altitude at dawn puts the ascents into a new perspective.

In the paper Twilight ascents by common swifts, Apus apus, at dawn and dusk: acquisition of orientation cues? the behaviour is described in detail, and alternative hypotheses are formulated regarding the functional significance of the ascents.

For further information also see this press release

Reflectivity altitude profile for the night of 1-2 June 2009, showing twilight ascents of Swifts (a). In addition to the dusk ascent, Swifts also participate in a dawn ascent. Both ascents are timed with respect to sunset and sunrise as each others mirror images. The dusk ascent shown in (b) has a shape almost identical to the time-inverted dawn ascent as shown in (c). The displayed time series starts at sunset and ends at sunrise; solid vertical black grid lines indicate the transition between civil twilight and nautical twilight; dotted vertical lines indicate the transition between night and nautical twilight.