Do penguins share? Evidence of foraging niche segregation between but not within two sympatric, central-place foragers

Niche theory predicts that sympatric species should differentiate ecologically in order to co-exist and conspecifics will also differentiate to reduce intra-specific competition. As central-place foragers, colonial breeding seabirds represent an ideal model system to test this theory and examine the mechanism of niche segregation. We used GPS-TDlog devices for tracking and diving data and stable isotope analysis to examine patterns of inter-and intra-specific niche segregation among southern rockhopper penguins Eudyptes chrysocome chrysocome and Magellanic penguins Spheniscus magellanicus breeding on Isla de los Estados, Argentina, across 3 consecutive breeding seasons. Tracking data indicated strong inter-specific spatial segregation of foraging locations and little overlap. Diving data also highlighted vertical foraging niche segregation as female rockhopper penguins dove deeper than male and female Magellanic penguins. delta C-13 values supported the general pattern of habitat segregation, with lower values for female rockhopper penguins that dove deeper and foraged off the shelf break. Female rockhopper penguins exhibited a lower relative trophic value (delta N-15) than male and female Magellanic penguins, consistent with previous dietary studies of both species. These differences likely act to reduce competition between the 2 species during the breeding season, when they are constrained to exploit the resources around their colonies. In contrast, male and female Magellanic penguins shared a similar foraging niche as measured by areas used to forage, dive depths, relative habitat use and trophic values. The lack of sex-specific foraging niche segregation of Magellanic penguins at Isla de los Estados could be related to the availability of food in the area and/or the small population size.