Diet studies of migratory birds have been a necessary tool for analyzing processes associated with overwintering site selection, interactions with primary prey species, detection of potential conflicts with anthropogenic activities and evaluation of habitat quality. The common tern (Sterna hirundo) is a neotropical migratory marine bird that breeds in North America and migrates to southern South America during the non-reproductive season. The region south of Samborombón Bay in Argentina hosts one of the largest non-breeding aggregations of this species during the austral summer. This project aimed to study aspects of common tern ecology that have yet to be addressed at the most important overwintering site in the region, using carbon and nitrogen stable isotope analyses.
The main objective was to evaluate the trophic ecology of common terns and its intraspecies variability in terms of (1) contribution of commercially important prey species to tern diets, (2) degree of fishing pressure by commercial fisheries and the associated level of risk to terns, (3) diet variability between the sexes, and (4) diet quality relative to individual body condition prior to leaving the overwintering grounds. Terns were captured using cannon nets during four field outings. Birds were measured and a blood sample was taken (<0.3 ml). Individuals were sexed genetically. Blood samples were dried, pulverized, weighed and encapsulated for stable isotope analysis.
There was no difference in stable isotope signatures between males and females, but there were differences in carbon and nitrogen values during different periods of the overwintering season. Differences in carbon levels could be due to exploitation of difference foraging resources and/or foraging areas in Samborombón Bay, although – especially early in the season – the differences also could be partly attributed to variability in migratory patterns. Meanwhile, a steady increase in nitrogen values suggests the consumption of prey of increasing trophic level over the course of the overwintering period. Full models of the stable isotope results are still under development and will more thoroughly address the project’s objectives. This information will be very useful for assessing the status of non-breeding groups of S. hirundo on their primary overwintering grounds, and it will provide critical input for developing a Samborombón Bay management plan. Finally, this project provided field and laboratory training and professional development for two undergraduate students who incorporated elements of the analyses into their senior theses. ($4,380)