Segregation of foraging between two sympatric penguin species: does rate maximisation make the difference?

Macaroni Eudyptes chrysolophus and gentoo Pygoscelis papua penguins occur sympatrically at Bird Island (54degrees00'S, 38degrees02'W), South Georgia, and have a similar diet. Macaroni penguins forage at shallower depths and further from the island than gentoo penguins. We tested the hypothesis that differences in the optimal foraging behaviour at the level of individual dives help to explain this ecological foraging segregation. Using a rate maximising diving model, we predicted that macaroni penguins would obtain more energy per dive cycle than gentoo penguins only when they used patches shallower than 30 to 40 m. This prediction was supported by observations of actual dive depths. We conclude that different foraging efficiencies at different depths results in each species being differentially sensitive to the vertical distribution of prey in the water column. We suggest that differing capacities to exploit natural heterogeneity in the distribution of prey has an important role in maintaining these 2 species as sympatric predators of a common resource.