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

We tracked by satellite the foraging trips of males and females of the two 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 as well as speed and distance covered on foraging trips were similar between species but lower for males than females in both species. Sex differences agree with previous observations on diet and on attendance at seal carcasses, suggesting that females mainly forage at sea, whereas males mainly scavenge on the coast. Overall, however, foraging ecology of both species seems very similar. Inter-specific competition may be reduced by the limited overlap in the activity range, with southern giant petrels foraging further south than northern giant petrels, suggesting some spatial partitioning in foraging areas. Male northern giant petrels foraged almost exclusively on the South Georgia coast; their strong dependence during the breeding season on fur seals, which have increased exponentially in recent years, may be ref1ected in their recent population increase at South Georgia. Foraging areas of giant petrels overlapped extensively with longline fishery distribution, confirming their susceptibility to being caught on longline hooks. Females were at higher risk during the study period since they made longer trips and further west than males, into areas where local longline fisheries are more active.