Marine Ecology Progress Series 308: 293-301 (2006)
Locomotion and foraging strategy in foot-propelled and wing-propelled shallow-diving seabirds.
Akiko Kato, Yan Ropert-Coudert, David Grémillet, Belinda Cannell
Buoyancy is a major determinant of locomotory cost in diving animals. As seabirds have
a large amount of air in their feathers and respiratory system, they should work hard against buoyancy
to descend in the water column. Since buoyancy decreases with increasing hydrostatic
pressure, shallow divers, especially, should work against buoyancy during both the descent and
bottom phases of their dives. We deployed miniaturised depth and acceleration recorders on freeranging,
foot-propelled divers (great cormorants Phalacrocorax carbo) and flipper-propelled divers
(little penguins Eudyptula minor) foraging in shallow waters, where both are positively buoyant. We
monitored the diving strategies of the birds to assess how they manage to counteract buoyancy using
2 alternative locomotory modes. Cormorants decreased the stroke frequency and surge amplitude
during descent and maintained constant swim speed. In contrast, penguins did not change their
stroke frequency or heave amplitude; as a result, their swim speed increased during the descent
phase. During the bottom phase, cormorants had a low stroke frequency and tilted towards the
bottom, while penguins actively stroked in a horizontal position. Furthermore, intensive acceleration
periods during the bottom phase were often recorded for penguins, but rarely for cormorants. Great
cormorants and little penguins used different strategies to feed underwater. Great cormorants,
having lower specific buoyancy, were less active than little penguins, thus adopting an energysaving
strategy. We conclude that both foot- and wing-propelled birds diving in shallow waters have
developed efficient locomotory strategies to counteract buoyancy, these alternative characteristics
being linked to different foraging niches.
BD06-03
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