Locomotion and foraging strategy in foot-propelled and wing-propelled shallow-diving seabirds

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 footand wing-propelled birds diving in shallow waters have developed efficient locomotory strategies to counteract buoyancy, these alternative characteristics being linked to different foraging niches.