Age-related prey selectivity and foraging efficiency in Olrog's Gulls Larus atlanticus feeding on crabs in their non-breeding grounds.

BERÓN PAULA; GARCIA GERMAN; LUPPI TOMAS; FAVERO MARCO

EMU

CSIRO PUBLISHING

Año: 2011

0158-4197

Abstract Foraging ecology is an essential component of the life history of a species, and a good understanding of foraging is especially important for threatened species whose prey populations may be susceptible to anthropogenic processes. This study was designed to study the age-related prey selection and foraging efficiency of Olrog´s Gulls feeding on crabs. The study was conducted in Mar Chiquita coastal lagoon, Argentina. Foraging behaviour and diet of Olrog´s gulls was determined by focal observation and, in order to analyse prey selectivity, contrasted with crab availability in foraging patches. All prey captured by gulls were crabs, with the Burrowing crab Neohelice granulata being a dominant prey over the Mud crab Cyrtograpsus angulatus. Gulls consumed small and medium crabs in larger proportions than available crabs. Also, gulls of all age classes consumed more male crabs of both species. Significant differences were found in handling times between age-classes, being significantly longer in juvenile´s gulls than older individuals. Handling times increased with crab size independently of the sex of prey. The handling efficiency in adults was significantly higher than subadults, which in turn was higher than juveniles. These differences in the trophic spectrum between age classes could be attributed to differential foraging skills and social subordination of juveniles. Large crabs might be avoided for other reasons that will need further exploration such as poor digestibility of carapaces, higher risk of injury or side effects of kleptoparasitic or social interactions.