Foraging dynamics of seabirds in the Eastern tropical pacific ocean

During a 9-yr period, 1983-1991, we studied the feeding ecology of the marine avifauna of the eastern tropical Pacific Ocean (ETP), defined here as pelagic waters from the coast of the Americas to 170° W and within 20° of the Equator. This is one of few studies of the diet of an entire marine avifauna, including resident breeders and non-breeders as well as passage migrants, and is the first such study for the tropical ocean, which comprises 40% of the Earth's surface. During spring and autumn, while participating in cruises to define the dynamics of equatorial marine climate and its effects on the seabird community, we collected 2,076 specimens representing, on the basis of at-sea surveys, the 30 most-abundant ETP avian species (hereafter; ETP avifauna). These samples contained 10,374 prey, which, using fish otoliths and cephalopod beaks, and whole non-cephalopod invertebrates, were identified to the most specific possible taxon. The prey mass consumed by the ETP avifauna consisted of 82.5% fishes (57% by number), 17.0% cephalopods (27% by number), and 0.3% non-cephalopod invertebrates (16% by number). Fish were the predominant prey of procellariiforms and larids, but pelecaniforms consumed about equal proportions of fish and cephalopods. Based on behavior observed during at-sea surveys, the ETP avifauna sorted into two groups-15 species that generally fed solitarily and 15 species that generally fed in multispecies flocks. Otherwise, the avifauna used a combination of four feeding strategies: (1) association with surface-feeding piscine predators (primarily tuna [Thunnus and Euthynnus spp.]), (2) nocturnal feeding on diel, vertically migrating mesopelagic prey, (3) scavenging dead cephalopods, and (4) feeding diurnally on non-cephalopod invertebrates (e.g., scyphozoans, mollusks, crustaceans, and insects) and fish eggs. Because of differential use of the four strategies, diets of the two seabird groups differed; the solitary group obtained most of its prey while feeding nocturnally, primarily on mesopelagic fishes (myctophids, bregmacerotids, diretmids, and melamphaids), and flocking species fed primarily on flying fish (exocoetids and hemirhamphids) and ommastrephid squid (Sthenoteuthis oualaniensis) caught when feeding diurnally in association with tuna. Many of the smaller species of solitary feeders, particularly storm-petrels, small gadly petrels and terns, supplemented their diets appreciably by feeding diurnally on epipelagic non-cephalopod invertebrates and by scavenging dead cephalopods. Flock-feeding procellariiforms also supplemented their diet by feeding nocturnally on the same mesopelagic fishes taken by the solitary species, as well as by scavenging dead cephalopods. Some spatial and temporal differences in diet were apparent among different species. An analysis of otolith condition in relation to hour of day that birds were collected showed that procellariiform species caught mesopelagic fishes primarily between 2000 and 2400 H. Selection of these fishes by size indicates that they occurred at the surface in groups, rather than solitarily. Solitary avian feeders had greater diet diversity than flock-feeders, particularly pelecaniforms. Appreciable diet overlap existed among the solitary and flock-feeding groups. Diet partitioning was evident within each feeding group, primarily exercised by using different feeding strategies and through selection of prey by species and size: larger birds ate larger prey. We classified five of the predominant ETP species, Sooty Shearwater (Puffinus griseus), White-necked Petrel (Pterodroma cervicalis), Murphy's Petrel (Pterodroma ultima), Stejneger's Petrel (Pterodroma longirostris), and Parasitic Jaeger (Stercorarius parasiticus), as migrants; based on stomach fullness, these species fed less often than the residents and were more opportunistic, using each of the four feeding strategies.