Intraspecific Host Selection by Kleptoparasitic Kelp Gulls in Patagonia

-We studied kleptoparasitic behavior of Kelp Gulls (Larus dominicanus) feeding on fishery waste at trawl vessels in northern Patagonia, Argentina, by experimentally discarding fish. Intraspecific kleptoparasitism by gulls of all age-classes was recorded at all hauls. During experimental discarding, Kelp 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 larger in younger age classes. The size of fish being carried by victims of kleptoparasitic 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 juveniles were successfully robbed irrespective of the size of the fish carried by them. Gulls of different age-classes were victims of kleptoparasitic attempts, in direct proportion to the number of fish handled by each age-classes. However, juveniles were successfully robbed more often than other age-classes. Juveniles lost prey in 83% of kleptoparasitic attempts, while sub-adults and adults lost their fish in 41 % and 42% of cases, respectively. Although selection of juvenile hosts to attack could be more profitable, attack rate on young birds was not higher than expected, suggesting attacks at trawlers were at random. Feeding where there is a high density of potential victims may be advantageous forjuveniles compared to feeding elsewhere because 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. Received 8 March 2000, accepted 8 May 2000.