Intraspecific host selection by kleptoparasitic Kelp Gulls in Patagonia

We studied kleptoparasitic behavior of Kelp Gulls (Larus dominicanus) feedi ng on fishery waste at trawl vessels in northern Patagonia, Argentina, by e xperimentally discarding fish. Intraspecific kleptoparasitism by gulls of a ll age-classes was recorded at all hauls. During experimental discarding, K elp Gulls attempted to steal fish from other gulls in 23.5% of 1.915 cases. The length of fish handled differed between age-classes, tending to be lar ger in younger age classes. The size of fish being carried by victims of kl eptoparasitic attempts was similar between age-classes. On the other hand. adult and sub-adult were more frequently attacked when they carried larger fish, while juveniles were victims of kleptoparasitic attempts independent of the size of the fish carried by them. The lengths of Successfully stolen fish were similar between victims of different age-classes. Adult and sub- adult were successfully robbed when they carried larger fish, while juvenil es were successfully robbed irrespective of the size of the fish carried by them. Gulls of different age-classes were victims of kleptoparasitic attem pts, in direct proportion to the number of fish handled by each age-classes . However, juveniles were successfully robbed more often than other age-cla sses, juveniles lost prey ill 83% of kleptoparasitic attempts, while sub-ad ults and adults lost their fish in 41% and 42% of cases, respectively. Alth ough selection of juvenile hosts to attack could be more profitable, attack rate on young birds was not higher than expected, suggesting attacks at tr awlers were at random. Feeding where there is a high density of potential v ictims may be advantageous for juveniles compared to feeding elsewhere beca use they are not selectively harassed or displaced by older, foraging gulls . If so, this could contribute to juvenile survival which could contribute to an increased population size.