Seabird associations in Pacific equatorial waters

Seabirds were counted along the same cruise tracks in the eastern tropical Pacific (10°øN-10°øS and 140°ø-90°øW) each spring and autumn for 8 years, 1984-1991. During spring, three species groups were apparent among eight species seen in five or more years: (1) Leach's Storm-petrel Oceanodroma leucorhoa and Wedge-rumped Storm-petrel Oceanodroma tethys, which feed on neuston by surface seizing; (2) Black-winged Petrel Pterodroma nigripennis and White-winged Petrel Pterodroma leucoptera, which feed on neuston and micronekton by surface seizing; (3) Juan Fernandez Petrel Pterodroma externa, Wedge-tailed Shearwater Puffinus pacificus and Sooty Tern Sterna fuscata, which feed on micronekton driven to the surface by foraging tuna. During autumn, only one group was identified from the seven species seen in five or more years: Leach's Storm-petrel associated with either Wedge-rumped Storm-petrel or Juan Fernandez Petrel and/or White-winged Petrel. Annual variability in assemblage composition was more extreme in the autumn and thus was similar to variation in physical and biological factors linked to the El Niño-Southern Oscillation cycle.