Foraging partitioning between giant petrels Macronectes spp. and its relationship with breeding population changes at Bird Island, South Georgia

Abstract

We satellite-tracked the foraging trips of males and females of the 2 sibling species of giant petrels, Macronectes halli and M. giganteus, breeding sympatrically at Bird Island (South Georgia, Antarctica), during the incubation period (November-December). Size of the activity range in addition to speed and distance covered on foraging trips were similar between the species, but were lower for males than for females in both species. Sex-specific differences agree with previous observations on diets and on attendance at seal carcasses, suggesting that females mainly forage at sea, whereas males mainly scavenge on the coast. Overall, however, the foraging ecology of both species seems very similar. Interspecific and intersexual competition may be reduced by the limited overlap in the at-sea range, with southern giant petrels foraging further south than did northern giant petrels, and females further west than males, suggesting some spatial partitioning in foraging areas. Male northern giant petrels foraged almost exclusively on the South Georgia coast; their strong dependence during the brooding and chick-rearing period on Antarctic fur seals, whose population has increased exponentially in recent years, may be reflected in the recent population increase of northern giant petrels at South Georgia. Foraging areas of giant petrels overlapped extensively with longline fishery distribution, highlighting their susceptibility to being caught on longline hooks. Females were at higher risk during the study period since they made longer trips and foraged further west than males, into areas where local longline fisheries are more active.