Elsevier

Animal Behaviour

Volume 85, Issue 3, March 2013, Pages 545-552
Animal Behaviour

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

https://doi.org/10.1016/j.anbehav.2012.12.006Get rights and content
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Common swifts are specialist flyers spending most of their life aloft, including night-time periods when this species roosts on the wing. Nocturnal roosting is preceded by a vertical ascent in twilight conditions towards altitudes of up to 2.5 km, behaviour previously explained as flight altitude selection for sleeping. We examined the nocturnal flight behaviour of swifts, as uniquely identified by a Doppler weather radar in central Netherlands using continuous measurements during two consecutive breeding seasons. Common swifts performed twilight ascents not only at dusk but also at dawn, which casts new light on the purpose of these ascents. Dusk and dawn ascents were mirror images of each other when time-referenced to the moment of sunset and sunrise, suggesting that the acquisition of twilight-specific light-based cues plays an important role in the progression of the ascents. Ascent height was well explained by the altitude of the 280 K isotherm, and was not significantly related to wind, cloud base height, humidity or the presence of nocturnal insects. We hypothesize that swifts profile the state of the atmospheric boundary layer during twilight ascents and/or attempt to maximize their perceptual range for visual access to distant horizontal landmarks, including surrounding weather. We compare twilight profiling by swifts with vertical twilight movements observed in other taxa, proposed to be related to orientation and navigation.

Highlights

► 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.

Keywords

Apus apus
atmospheric assessment
celestial cue
common swift
flight altitude
orientation
navigation
twilight
weather radar

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