Offshore spatial segregation in giant petrels Macronectes spp.: differences between species, sexes and seasons

1. Investigations were made to determine whether the two giant petrel species segregate by gender and species in relation to the stage of the annual cycle. The individual foraging behaviour of 14 male and 11 female northern giant petrels (Macronectes halli) and 13 male and 15 female southern giant petrels (M. giganteus) breeding at South Georgia were tracked over 1 year using geolocators (global location sensing loggers). 2. Males of both species showed a flexible foraging strategy, switching from coastal to pelagic habits, probably governed by spatio-temporal changes in carrion availability. In contrast, marine areas exploited by females were more consistent over the year and similar for the two species, with most foraging locations concentrated over the same pelagic waters. 3. This study provides support for the differences in foraging between sexes as the main mechanism reducing intraspecific competition. Although the two species are morphologically similar and can easily access each other's foraging habitat, they differ in the foraging areas exploited. Thus, interspecific competition seems mainly relaxed by spatial segregation, particularly between males in winter, probably mediated by different competitive abilities and physical tolerances to temperature and winds. Foraging southern giant petrels from South Georgia were not restricted to the areas within the line of equidistance to other colonies, but their foraging range overlapped with feeding grounds of conspecifics breeding in the Falkland Islands and the Antarctic Continent. 4. Taken together, these findings suggest that foraging selection on marine habitat heterogeneity reduces interspecific competition, whereas carrion availability reduces intersexual competition, in giant petrels.

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