Effects of age, sex, colony and breeding phase on marine space use by Great Shearwaters Ardenna gravis in the South Atlantic

Shearwaters are among the most abundant seabirds globally and breeding birds often travel thousands of kilometres during foraging trips to productive marine areas. Great Shearwaters Ardenna gravis are endemic breeders of the Tristan da Cunha Archipelago with a population of 5–6 million breeding pairs making them key top predators within the South Atlantic Ocean. We deployed satellite transmitters on 42 breeding and non-breeding shearwaters at two nesting islands and one foraging site in the northern hemisphere to quantify their movements and space use with respect to age, sex, colonies and breeding phases. During the pre-laying period, birds from Gough Island made trips of 2400 km to the Benguela upwelling region off South Africa, or > 4000 km to the Patagonian Shelf; patterns which overlapped with immature birds from the Bay of Fundy, Canada. During the incubation period, males and females from Inaccessible Island showed differences in trip durations, but no difference in foraging ranges: both sexes made trips of 1500–2000 km to the Sub-Antarctic Front or 4000 km to the Patagonian Shelf, among the furthest foraging trips of breeding seabirds observed to date. Although the colonies are separated by only 400 km, during the incubation period, the at-sea distribution of non-breeding birds from Gough Island was spatially segregated from breeding and failed birds from Inaccessible Island, whereas immature birds from Fundy overlapped with birds from both colonies. During the post-breeding period, all tagged populations overlapped on the Patagonian Shelf. Argentina’s Exclusive Economic Zone was used extensively during pre-laying, incubation, and post-breeding periods, highlighting the global importance of the Patagonian Shelf for this species.