Inferences on seabird ecology from stable isotopes ratios (δ13C, δ15N) and mercury concentrations analysis of feathers have been made for temperate and polar species but are far more rare for tropical species. In this paper, we used this approach combined with analysis of regurgitations and feeding observations at colonies to examine diet segregation between Roseate Terns (Sterna dougallii) and Lesser Noddies (Anous tenuirostris) breeding sympatrically on Aride Island (Seychelles), western Indian Ocean. Our results indicated extensive overlap between the two species in trophic level and foraging area during the breeding season. Goatfish predominated (93-97%) in all diet samples of adults and chicks collected in the colonies, except in prey fed to mates by Roseate Terns, of which scad and tuna comprised 20%. The isotopic analyses of feathers replaced by adults during molt (primary and body feathers) suggested, however, that the two species differ in foraging ecology during the nonbreeding period. Roseate Tern adults had consistently lower δ15N values than Lesser Noddies which, in turn, had δ15N values comparable to those of chick feathers grown on Aride. Moreover, low but similar mercury levels were found in body feathers of Lesser Noddy adults and Roseate Tern chicks, whereas Roseate Tern adults were significantly more contaminated. Overall, these results support the hypothesis that the Lesser Noddy is largely sedentary, being associated with the same food web in the vicinity of the colonies year-round. In contrast, Roseate Terns rely on distinct prey during the molting (nonbreeding) season which may be also consistent with a change in food web (i.e., a migratory regime) although the assignment of potential wintering areas remain difficult without isotopic basemaps currently available for the Indian Ocean.
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Vol. 31 • No. 2