Where two oceans meet:: distribution and offshore interactions of great-winged petrels Pterodroma macroptera and Leach's storm petrels Oceanodroma leucorhoa off southern Africa

During seabird surveys off southern Africa, great-winged petrel Pterodroma macroptera and Leach's storm petrel Oceanodroma leucorhoa were widespread and relatively common. Similar aerial displays, frequent (social) interactions, a tendency to "huddle" in tight (mixed) gatherings and interactions during foraging inspired a study of their behaviour and distribution. Both species peaked at > 2,000-m-deep ocean waters, with lower densities over the shelf and with Leach's storm petrels (ca. 0.5 km(-2)) twice as abundant as great-winged petrels (ca. 0.25 km(-2)). The results suggest half a million great-winged petrels and well over a million Leach's storm petrels occurring over deep waters off southern Africa. Active fishing vessels elevated background densities of petrels in some areas, indicating the utilisation of discarded material, but in most areas hardly any fisheries were encountered and on the shelf with no discernable effect on petrel densities. Both species showed an association with meso-scale hydrographic features in the Agulhas current retroflection region. High densities of Leach's storm petrels occurred in offshore areas with steep salinity and sea surface temperature gradients. No such tendency was found in great-winged petrels. Great-winged petrels spent relatively little time feeding during daylight (10.3% of birds observed) in comparison to Leach's storm petrels (66.2%), but were frequently seen to join feeding Leach's storm petrels where they profited from the searching skills of Leach's. Both species performed displays as individuals, in pairs, in larger groups and in mixed-species groups. Tight gatherings of petrels swimming at sea (huddling) occurred, consisting either of one species or both. When disturbed, such flocks would disperse after take off and often engage in aerial displays. It is speculated that the formation of these flocks is part of the anti-predator strategy of petrels against attacks from under water.