Among seabirds, Terns are particularly sensitive to food shortages because their energy reserves are small and the time required for foraging is over. In Brazil, the Terns diet at breeding colonies has been reported in Santa Catarina, while investigations into the feeding ecology of common terns (Sterna hirundo) using regurgitated pellets in wintering areas were carried out in Rio Grande do Sul and Punta Rasa, Argentina. The use of stable isotopes allows to check the relative contributions of marine and terrestrial foods in the diets of various species by carbon, trace the origins of migratory animals of breeding and wintering using deuterium, carbon and nitrogen and the impact of human activities. The isotopic approach does not replace conventional techniques (direct observation, pellets, regurgitation, telemetry, etc.), where detailed information is obtained, particularly when several diet options are available. However, important advantages in the use of stable isotopes in studies of the diet include the estimation of food assimilated and not just eaten, and achieving long-term comparative information. Feathers and blood are increasingly used by ecologists for the study of stable isotopes in birds. Besides being a non-destructive method, the feathers are formed at different times of year and can potentially provide access to information feed covering different scales of space and time. In Brazil the use of stable isotope analysis of diet in seabirds is still incipient and this technique being used in Argentina will assist understanding of the trophic web of species that breed on the coast of São Paulo and Santa Catarina (Sterna hirundinacea, Thalasseus acuflavidus, Thalasseus maximus) and use the wintering areas on the beaches of São Paulo, Santa Catarina and Rio Grande do Sul for food and rest. Two species of terns, T. maximus and S. hirundinacea are considered endangered in the State of São Paulo, according to State Decree 42.838/98, while T. maximus is considered endangered in Brazil. The hypothesis is that the signatures of ´13C and ´15N in feathers and blood samples obtained from breeding sites are different from the rest areas by providing a tool for management and conservation of these species, since food is one of the factors limiting factors for reproductive success and any changes in ecological conditions in rest areas and migration can result in harmful consequences in the way of life and body condition.
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