Age-related feeding behaviour and foraging efficiency in Kelp Gulls Larus dominicanus attending coastal trawlers in Argentina

We studied age-related differences in feeding behaviour and foraging succes s of Kelp Gulls Larus dominicanus feeding on fishery discards at trawl vess els in northern Patagonia, Argentina. Kelp Gulls consumed fish by direct ca pture and intraspecific kleptoparasitism. Direct capture rate increased sig nificantly with age. Juveniles dropped a higher proportion of handled fish than older birds. Intraspecific kleptoparasitism involved gulls of all age- classes and was recorded in all hauls. The percentage of successful attempt s was similar among different age-classes. However, juveniles and immatures attempted to steal prey more frequently and therefore obtained more prey t han expected. The proportion in which each feeding method (direct capture a nd kleptoparasitism) contributed to overall fish consumption varied signifi cantly among age-classes. Juveniles obtained most of the food through klept oparasitism (72%), while immature, subadult and adults consumed prey mostly through direct capture (77, 91 and 92%, respectively). Using both feeding methods, Kelp Gulls swallowed 87.6% of the discarded fish that they handled . However the percentage of fish swallowed over total handled fish were dif ferent among age-classes (juveniles 45.5%, immatures 70.8%, subadults 83.3% , adults 94.8%). Kleptoparasitism may be used as an alternative strategy to compensate the lower efficiency of young birds.