Kleptoparasitism by Kelp Gulls on Royal and Cayenne Terns at Punta Leon, Argentina

Several studies have shown that terms can be affected by kleptoparasitism f rom gulls. During 1992 and 1993 we studied kleptoparasitism at a mixed-spec ies colony of Kelp Gulls (Larus dominicanus), Royal Terns (Sterna maxima), and Cayenne Terns (S. eurygnatha) at Punta Leon, Chubut, Argentina. Robbing behavior did not involve aerial chases. Kelp Gulls remained at the ternery periphery and stole food from adults flying low over the colony with food intended for chicks, from adult terms on the ground transferring food to th eir chicks, or from chicks. The percentage of food robbed by Kelp Gulls fro m both tern species was 8.8% of food brought to the colony (n = 1131). Robb ing efficiency was 58%. Kelp Gulls robbed significantly more food from Roya l than Cayenne Terns (10.6 vs. 3.6%, respectively). The percentage of robbi ng attempts was also larger on Royal than Cayenne Terns (18.9 vs. 5.7%). Ro bbing efficiency, however, was similar for both tern species (56 and 63.2% for Royal and Cayenne Terns, respectively). Kleptoparasitism on Royal Terns within the colony was related to the age of their chicks, being higher on terns with chicks older than 20 days of age. The risk of a Royal Tern being robbed by Kelp Gulls was larger when it was feeding chicks at the colony t han on the beach (14.9% vs. 4.5%), although differences observed were not s ignificant.