Comparative foraging ecology and ecological niche of a superabundant tropical seabird: the sooty tern Sterna fuscata in the southwest Indian Ocean

Over 6-million pairs of sooty terns Sterna fuscata breed once a year in the southwest Indian Ocean, mostly on three islands of the Mozambique Channel (Europa, Juan de Nova and Glorieuses) and in the Sey- chelles region. Seasonal reproduction in either winter or summer is the dominant strategy in the area, but non- seasonal reproduction also occurred in some places like at Glorieuses Archipelago. The feeding ecology of the sooty tern was investigated during the breeding seasons to deter- mine whether terns showed signiWcant diVerences in their trophic ecology between locations. Regurgitations were analyzed to describe the diet of individuals when breeding, and stable isotopes and mercury concentrations were used to temporally integrate over the medium-term of the trophic ecology of both adults and chicks. Overall, the diet was composed of Wsh, Xying squid and Wsh larvae in diVerent proportions. At Europa and Aride in the Seychelles, where winter reproduction occurs, large epipelagic prey like Xying Wsh or squid dominated the diet. At Juan de Nova, sooty terns reproduce in summer and rely mostly on Wsh larvae. At Glorieuses (non-seasonal breeding), the diet was inter- mediate with Wsh larvae and Xying squid being important prey items. The stable-carbon and nitrogen isotope values in blood conWrm the diVerences observed in dietary analy- sis, and demonstrate diVerent feeding strategies between colonies. 13 C values of feathers showed spatial segrega- tion between birds from the Mozambique Channel and the Seychelles region. Terns from the Seychelles had also higher 15 N values. Feather 13 C values also suggest a sig- niWcant shift from summer to wintering habitat for birds from Juan de Nova. This study emphasizes the high pheno- typic plasticity of the species, which may explain its numerical dominance in all tropical waters of the World's Ocean.