DIET COMPOSITION AND SEASONAL CHANGES IN PREY SELECTION IN THE THREATENED OLROG?S GULL LARUS ATLANTICUS BREEDING IN NORTHERN PATAGONIA, ARGENTINA

NICOLÁS SUÁREZ; MA. VALERIA RETANA; PABLO YORIO

Victoria

Congreso; 1st World Seabird Conference; 2010

Knowledge of seasonal changes in resource use is important to adequately understand predator-prey relationships. We assessed seasonal changes in prey selection in the crab-eating Olrog?s gull. Diet was assessed through pellet analysis (n=360) during 2006 and 2007. Prey availability was assessed by sampling 1540 1m2 quadrates distributed throughout potential feeding areas. Gulls fed almost exclusively on three crabs: Neohelice granulata, Cyrtograpsus altimanus, and C. angulatus. The relative importance (IRI%) of different crab species varied throughout the breeding cycle. During incubation, N. granulata showed the largest contribution (>95%) while C. altimanus contributed with less than 2%. This relative contribution was reversed during the early chick stage (60%, respectively) and was between 40 and 50% for both species during the late chick stage. C. angulatus showed relatively small %IRI values throughout the cycle. For each crab species, consumed sizes were similar among the three stages (P>0.05). Percent occurrence, numerical abundance, and density of available crabs was similar between N. granulata and C. altimanus, and smaller in C. angulatus (P