November
19, 2018, The Company of Biologists
Once an
adult swift (Apus apus) leaves its
breeding colony and takes to the air migrating south, it won't touch down again
until returning home to nest 10 months later. "Common swifts are
exceptional in their level of adaptation to aerial life," says Emmanuel de
Margerie, a biologist from the CNRS (Centre National de la Recherche
Scientifique) at the University of Rennes, France, adding, "Foraging, sleeping,
preening and all other daily activities are performed in mid-air, day after
day, week after week." So, when de Margerie decided to learn how the
expert aviators manoeuvre in their aerial domain, he contacted biomechanist
Tyson Hedrick from the University of North Carolina at Chapel Hill, USA, who
snapped up the opportunity. "Their basic flight capabilities have been
well studied in wind tunnel experiments," says Hedrick. However, birds in
wind tunnels never share the sky with others or contend with unexpected gusts
of wind. de Margerie had filmed swifts soaring and swerving while foraging to
feed their chicks and the movies provided the ideal opportunity to find out how
much exertion it takes to keep an acrobatic swift on the wing in real life.
They publish their discovery that swifts essentially hitchhike on rising
currents to make their flight costs almost zero in Journal of Experimental
Biology.
But the
unique footage was not collected with a conventional camera. Mounting a pair of
angled mirrors either side of a camera, with a third mirror in front of the
lens to collect the reflections from the wide-set mirrors, de Margerie was able to film a pair of simultaneous movies—each from a
slightly different perspective—which
he could then analyse to perfectly reconstruct individual swift motions in 3-D.
"The current device is cumbersome," says de Margerie, admitting that
it takes time to learn how to track the swifts' tortuous flight paths.
"After some training, our undergraduate student Cécile Pichot was the most skilful at continuously
following foraging swifts for relatively long flight times of up to 6
minutes," he says.
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