Breeding seasons of seabirds at Europa Island (southern Mozambique Channel) in relation to seasonal changes in the marine environment

Tropical seabirds have evolved a wide range of breeding regimes to fit with their local environment. The western Indian Ocean shows various oceanic si tuations, providing opportunities to link variations in breeding phenology with variations of the environment. The breeding seasons of seabirds at Eur opa Island (Mozambique Channel, western Indian Ocean) are analysed. Three o ceanic, tuna-associated species (the sooty tern Sterna fuscata, the red-foo ted booby Sula sula, and Audubon's shearwater Pyffinus lherminieri) breed d uring austral winter (June-October), whereas the red-tailed tropicbird Phae thon rubricauda breeds during austral summer (November-April). During winte r, sea-surface temperatures are the lowest, subtropical waters are the clos est to the island, and long-line catches of tunas are important. Thermal fr onts occur during winter, in relation to the northward shift of subtropical waters. This is known to be favourable to tunas, and I suggest that the wi nter breeding of sooty terns, red-footed boobies and Audubon's shearwaters is because of an increase in the abundance of tunas. Geographic variations in the breeding phenology of sooty terns in the western Indian Ocean largel y support the hypothesis that sooty terns take advantage of cool subtropica l waters, when those waters are within their foraging range. The red-tailed tropicbird is a solitary forager, not associated with tunas. During the ch ick-rearing period, 40% of the mass of food brought to the chicks consists of dolphin-fishes Coryphaena spp. Those warm water, surface dwelling fishes are known to migrate in relation to sea-surface temperatures. Seasonal cha nges in the occurrence or vulnerability of dolphin-fish in the Mozambique C hannel may have favoured a summer breeding regime in red-tailed tropicbirds at Europa Island. Further studies of the marine life of seabirds are neede d to test these hypotheses and to learn more about the foraging ecology of tropical seabirds.