Assessing the trophic niche of South American Terns integrating conventional and isotopic methods

Abstract

Conventional dietary studies combined with stable isotope analysis allowed the characterisation of the trophic niche of South American Terns (Sterna hirundinacea) breeding in Argentina. Direct observation of prey deliveries during mate- and chick-provisioning showed that the diet of South American Terns comprised at least 18 prey items, mainly pelagic schooling fish with smaller proportions of marine invertebrates. Argentine Anchovy (Engraulis anchoita) was the main fish prey. Adults delivered a higher proportion of fish and smaller prey to their chicks than to their mates. Analysis of stable isotope ratios in blood samples, using Bayesian stable isotope mixing models, indicated that adult Terns used a higher proportion of marine invertebrates for self-feeding than for mate- or chick-provisioning. Analyses of stable isotopes ratios in blood samples from chicks were consistent with the determination of diet from direct observation, and indicated differences from adults in both the proportion of different prey in the diet and trophic niche. In addition, isotopic niche analysis showed that chicks had a significantly higher trophic position compared with self-feeding adults during both mate-provisioning and chick stages. The results highlight the importance of combining stable isotope and conventional methods to adequately assess the food requirements of a population during the breeding season.