http://www.sciencedaily.com/releases/2013/02/130221084709.htm
Common Swifts Make Mysterious Twilight Ascents
Feb. 21, 2013 — Common swifts climb to altitudes of up to 2.5 km both
at dawn and dusk. This unexpected behaviour was discovered by geo-
ecologist Dr Adriaan Dokter of the University of Amsterdam's (UvA)
Institute for Biodiversity and Ecosystem Dynamics (IBED) together with
colleagues from the Royal Netherlands Meteorological Institute (KNMI),
the Royal Netherlands Air Force and Lund University.
The research results were published as a featured article in the March
issue of the scientific journal Animal Behaviour.
In the summer, swifts make their presence felt in the city, especially
because of the screaming sound they make. These common urban birds
appear to have unusual movement patterns. Besides a short breeding
period under urban roofing tiles, these birds spend all year on the
wing: foraging, mating and even sleeping are all performed in flight.
Using a novel radar technique, Dokter and his colleagues studied the
nocturnal flight behaviour of the common swift with the assistance of
a weather radar belonging to the KNMI.
Dokter explains: 'We always assumed that common swifts ascended in the
evening as part of their sleeping cycle. However, now that we have
observed the bird perform ascents at both the start and the end of
night-time periods, this interpretation seems to be incorrect. The
ascents must have a different function, with the birds making use of
the unique opportunities offered by the twilight period.'
Common swifts as weather predictors?
Twilight is rich in information: it is the only period allowing
simultaneous detection of landscape features, polarization patterns,
stars and magnetic cues. This information is used by many different
animals for navigational and orientation purposes. Common swifts also
seem to choose the twilight to obtain specific information during
their ascents. An appealing possibility is that during the ascents,
the birds investigate characteristics of the atmosphere in relation to
their current location and orientation, such as temperature and wind
at various altitudes. This may enable the birds to predict future
weather conditions. An adequate response to meteorological conditions
is crucial to common swifts, as when foraging they are fully dependent
on aerial insects, which are most common in good weather conditions.
Journal Reference:
• Adriaan M. Dokter, Susanne Åkesson, Hans Beekhuis,
Willem Bouten, Luit Buurma, Hans van Gasteren, Iwan Holleman. Twilight
ascents by common swifts, Apus apus, at dawn and dusk: acquisition of
orientation cues? Animal Behaviour, 2013; DOI:10.1016/j.anbehav.
2012.12.006
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