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Within seabirds, terns are frequently affected by foodrobbing. We document the occurrence of kleptoparasitism at a mixed-species colony ofRoyal (Sterna maxima) and Cayenne (S. eurygnatha) terns at Punta León, Chubut. In two seasons we observed both inter- and intra-specific food robbing by adult terns. The amount of food lost to pirates was relatively low, only 2% of prey brought back by the terns (n = 1127). Intra-specific food robbing was higher than inter-specific kleptopatasitism in the Royal Tern and similar in the Cayenne Tern. The percentage of food lost topirates in thecolony(2.1%,n = 921)wassimilarthan in groupsoftem chicksat the beach(2.4%, n = 206). Kleptoparasitismbetweentems at PuntaLeón appears to be of 1ittle importance with respect to its effectsonbothtem populations.

Conocer la sensibilidad de las especies al disturbio humano puede ayudar en la evaluación del impacto sobre las colonias de aves marinas. Durante 1989 a 1991 evaluamos la respuesta a la presencia humana, de 5 especies de aves marinas en la colonia mixta de Punta León, recientemente propuesta como reserva turística. La distancia a la cual los Cormoranes Imperiales (Phalacrocorax atriceps) comenzaron a responder a los acercamientos decreció a lo largo de la estación reproductiva (l00 a 6.6 m). El abandono de nidos periféricos ocurrió en todos los acercamientos a menos de 8 m y afectó exclusivamente la sección próxima al disturbio. Los huevos expuestos fueron predados por Gaviotas Cocineras (Larus dominicanus). Durante el asentamiento, algunos Biguás (Phalacrocorax olivaceus) abandonaron sus nidos cuando nos aproximamos a menos de 25 m, mientras que durante la incubación, los comportamientos de alerta ocurrieron a distancias menores de 20 m. Las distancias a las cuales las Gaviotas Cocineras levantaron vuelo en respuesta a la presencia humana, disminuyeron a medida que avanzó la temporada (>100 a 6.7 m). Durante los acercamientos a la colonia de Gaviotines Reales (Sterna maxima) y Pico Amarillo (S. eurygnatha), tanto los individuos que se estaban asentando como los que estaban incubando en la periferia, comenzaron reaccionar a distancias entre 20 y 25 m de la colonia. El abandono de nidos por parte de los gaviotines resultó en predaciones de huevos por Gaviotas Cocineras en tres de cuatro acercamientos. Las medidas presentadas son estimaciones mínimas que podrían ser usadas como referencias al evaluar las distancias óptimas de acercamiento a las colonias a fin de minimizar el disturbio. Palabras clave: aves marinas, colonias mixtas, disturbio humano, turismo, Patagonia. Knowledge ofspecies sensitivity to disturbance may aid in the evaluation of potential impacts of human activity on seabird colonies. During 1989 to 1991 we evaluated the response of 5 seabird species to human presence at the Punta León mixed-species colony, a site currently proposed as a tourist reserve. The response of all species varied throughout the season. The distance at which Imperial Cormorants(Phalacrocorax atriceps) started to respond to human approach decreased as the season advanced (100 to 6.6 m). Nest abandonment ofperipheral nesters occurred during a11 approaches at les s than 8 m and only near the section of the colony approached. Exposed eggs were preyed upon by Kelp Gu11s (Larus dominicanus). During settlement, some Olivaceous Cormorimts (Phalacrocorax olivaceus) abandoned their nests when approached to less than 25 m and, during incubation, alert behavior was observed at less than 20 m. Distance at which Kelp Gu11sstarted to flush in response to human approach also decreased as the season advanced (> 100 to 6.7 m). During the approach to the Royal (Sterna maxima) and Cayenne Tem (S. eurygnatha) colony, both settling and incubating peripheral individuals started to react with low intensity at approximately 20-25 m. Nest abandonment of incubating tems resulted in egg predation by Kelp Gu11in three of the four approaches. The distances presented are minimum estimates which could be used as references when deciding the optimal distances at which visitors should be kept away in order to minimize disturbance to the breeding birds.