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The foraging behaviour of animals depends on the distribution, abundance and predictability of their food resources. In the marine environment, top predators such as seabirds are known to concentrate their foraging effort in specific oceanic features where productivity is elevated or prey concentrated. When marine productivity is low and prey distribution unpredictable, such as in tropical waters, selection should favour the evolution of flexible foraging strategies. By using GPS, Argos transmitters and activity recorders, we studied the foraging strategy of red-footed boobies (RFBs) Sula sula breeding on Europa Island in the Mozambique Channel, to examine the way a central place forager searches for prey in tropical waters. RFBs only foraged during the day-time and never sat on the water at night; thus, they consequently had a limited maximum foraging range of 148 km. Primary production within this range was low overall (average 0.165 mg chlorophyll m -3 ), with a high spatial and temporal heterogeneity due to the presence of strong geostrophic currents and surface mixing by storms. RFBs appear to adjust their foraging strategy to cope with this situation. Individual birds do not return to the same oceanic sites from one trip to the next, but generally head in the direction of zones where productivity is enhanced. Active foraging, i.e. periods when birds spent time on the sea surface or dived, occurred at the most distant part of the foraging trip. Despite their limited foraging range and the low productivity of surrounding waters, red-footed boobies are able to track zones of prey availability located in areas of enhanced primary production. Searching is probably favoured by the presence of sub-surface predators that chase prey to the surface. The foraging strategy of this tropical sulid appears to differ extensively from that of temperate or polar seabirds whose prey distribution and availability are more predictable.

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In tropical waters, productivity is low and seabirds only feed at the surface, often in association with sub-surface predators. We studied the distribution and abundance of seabirds in the coastal marine environment around Réunion Island, SW Indian Ocean, between February 2001 and October 2002. We investigated the distribution and abundance of birds at sea with respect to the effects of natural factors, such as bathymetry, distance to the shore, season and associations with subsurface predatory schools, and the presence of anchored fish aggregating devices (FADs). A total of 13 572 birds of 15 species were observed, 8 of which were rare. Among the 7 abundant species, lesser noddies appeared to favour the shelf area. Barau's petrels and sooty terns were more abundant offshore, and wedge-tailed shearwaters, Audubon's shearwaters, white-tailed tropicbirds and brown noddies were present over all habitats. When foraging, most species searched for prey solitarily or in very small groups. When feeding, they were generally in large multi-species flocks, especially when feeding events occurred in association with sub-surface predatory schools. Associations with surfacefeeding schools of tunas, mainly skipjack, increased the abundance of most seabird species that fed on small fish or squids. The notable exception was for the white-tailed tropicbird, which was rarely found in association with sub-surface predators and, when associated, preferentially selected dolphin pods rather than tuna schools. Although they were often associated with tuna schools, lesser noddies were also strongly concentrated on the shelf, feeding on small pelagic schools. While the aggregating power of FADs on pelagic fish has previously been demonstrated, the associations between schools of sub-surface predators and seabirds were not more frequent close to FADs. However, 2 seabird species were more abundant close to FADs when feeding, and 1 when foraging, suggesting that they may occasionally take advantage of the attractive power of FADs. Seasonal variations in the occurrence of associations indicated a turn-over of fish species that induced variations in the nature of the associations, with the greatest aggregating power of dolphin fish in spring during their peak of abundance in the vicinity of the island.