Temporal changes in diet and prey selection in the threatened Olrog's gull Larus atlanticus breeding in southern Buenos Aires, Argentina.

We examined temporal changes in diet composition and prey selection in the threatened Olrog's gull Larus atlanticus breeding at Bahía San Blas, Argentina. Diet was assessed through pellet analysis (N = 360) and chick stomach samples (N = 120) during 2006 and 2007. Prey availability was sampled in 2,084 1m² quadrats distributed throughout Olrog's gull potential feeding areas. Gulls fed almost exclusively on three crabs: Neohelice granulata, Cyrtograpsus altimanus and Cyrtograpsus angulatus. Diet composition was similar between years. The relative importance (%IRI) of different crab species, assessed through pellet analysis, varied across the breeding cycle. During incubation, N. granulata predominated (> 90%) while C. altimanus comprised less than 2% of the crabs taken. This relative contribution was reversed during the young chick stage (< 10% v. > 60%, respectively) and was 40–50% for both species during the old chick stage. Stomach sample analysis provided a similar pattern of %IRI values for the chick stages. Sizes of available crabs differed significantly between species, C. angulatus and N. granulata both being similar-sized but larger than C. altimanus. Gulls selected N. granulata during incubation and largely C. altimanus during the chick stages. Our study suggests that Olrog's gull depends on three crab species, confirming its specialised feeding ecology during the breeding season. Their relative consumption, however, can change temporally, probably in response to restrictions imposed by the different requirements of adults and chicks.