Pedalling downhill and freewheeling up; a penguin perspective on foraging

Depth-dependent buoyancy resulting from the compression of body-associated air is a major force modulating energy expenditure in diving seabirds, yet quantification of its effects in free-living animals is problematic. Between November 2006 and December 2008, we used multiple channel loggers (daily diaries [DDs]); recording triaxial acceleration, depth and speed, during foraging of 34 Magellanic penguins Spheniscus magellanicus from 3 colonies in Argentina to derive a new proxy for energy expenditure, overall dynamic body acceleration (ODBA). Assuming ODBA to be linearly related to power, energy expenditure was highest during dive descent, nearer the surface and in those dives terminating at greater depths due to steeper descent angles. Swim speed during descent was invariant of maximum dive depth. Calculated energy expenditure during the bottom phase of the dive was invariant of depth, but energy expenditure of birds returning to the surface was lowest at any given depth for birds that had engaged in deeper dives due to the effects of buoyancy. Four birds, equipped with beak angle-measuring sensors as part of the DDs, captured 89% of their prey (normally pelagic school fish) during fast passive ascents using buoyancy to aid in capture. Details from one of these birds showed that this passive ascent occurred at a mean velocity of 1.94 m s super(-1) for a mean of 2.02 s; ascent angles during such rushes were steeper when the bird was deeper, which was presumed to reflect a response to the diminished buoyant force at greater pressures. Passive ascents during prey capture appear to be an important mechanism for Magellanic penguins to capture fast-moving prey with minimal energy expenditure.