Diet composition and seasonal changes in prey selection in the threatened Olrog’s Gull Larus atlanticus breeding in Northern Patagonia, Argentina

SUÁREZ, N.; RETANA, M.V.; YORIO, P.

Victoria

Congreso; First 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 (<10% vs. >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<0.05). Sizes were larger in N. granulata and C. angulatus (P<0.05). The Savage index showed that gulls selected N. granulata during incubation and largely C. altimanus during the chick stages. During incubation, gulls selected ‘medium’ and ‘large’ sized crabs, avoiding ‘small’ available crabs. In contrast, ‘small’ crabs were selected during both chick stages. Gulls chose crab species that were more available, switching from N. granulata to the smaller C. altimanus very likely as a result of constraints imposed by small chick prey requirements.