Where are you from? Isotopic tracing of juvenile Olrog’s Gulls from Mar Chiquita during the wintering season

ABSTRACT Individual traits such as age-class can have profound effects on space utilisation by individual birds, with juvenile birds more often exhibiting dispersive movements. We studied the origin of juvenile Olrog’s Gulls Larus atlanticus, a threatened species, using stable isotopes ratios of carbon (δ13C), nitrogen (δ15N) and sulphur (δ34S). We analysed feathers from individuals captured in the wintering season at Mar Chiquita Coastal Lagoon, Argentina, and the potential trophic sources of their breeding sites at Bahía Blanca and San Blas. Mean δ13C was similar for trophic sources and mean δ15N was higher for Bahía Blanca. The δ34S showed a greater variation among prey species than among sites. The mean contributions of the main colonies resulted in assigning 37.5% individuals as from Bahía Blanca, 16.7% from San Blas, and 45.8% undetermined. Results indicate that Mar Chiquita coastal lagoon attracts juvenile Olrog’s Gulls dispersing from the two main breeding grounds of the species. This work is one of the first approaches to study the origin of dispersive juvenile larids through the application of stable isotope analysis. This isotopic approach allows obtaining trophic and movement information when it is not possible to use other tools.