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John M. Maniscalco, William D. Ostrand, Robert M. Suryan, David B. Irons
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We analyzed data from two independent studies of foraging Black-legged Kittiwakes (Rissa tridactyla) in Prince William Sound, Alaska. Our purpose was to determine if Glaucous-winged Gulls (Larus glaucescens) hindered prey capture by kittiwakes. At tightly aggregated feeding flocks, gulls remained on the water directly over the prey and foraged by making brief hop-plunges or surface-seizes. Kittiwakes, in contrast, fed by diving from the air into open spots in the flock or around its periphery. Data from both studies indicated that kittiwakes made fewer feeding attempts in flocks that had greater numbers of gulls. However, kittiwake success rate per feeding attempt did not change as the number of gulls increased. Kittiwakes were more likely to avoid flocks that had a greater number of Glaucous-winged Gulls. Gulls successfully pirated less than 1% of fish captured by kittiwakes. Our findings suggest that passive interference may be costly for smaller birds that feed in multispecies feeding flocks.

Competencia por Interferencia Pasiva de Larus glaucescens sobre Rissa tridactyla: El Costo del Forrajeo en Grupo

Resumen. Analizamos datos de dos estudios independientes sobre forrajeo de Rissa tridactyla en Prince William Sound, Alaska. El propósito de nuestro estudio fue determinar si Larus glaucescens impedía la captura de presas por parte de R. tridactyla. Las gaviotas permanecieron sobre el agua directamente sobre sus presas en bandadas de forrajeo altamente agregadas y forrajearon realizando zambullidos breves o capturas de superficie. En contraste R. tridactyla forrajeó lanzándose desde el aire a espacios abiertos entre la bandada o en su periferia. Los datos de ambos estudios indicaron que R. tridactyla realizó menos intentos de forrajeo en bandadas que tenían mayor número de gaviotas. Sin embargo, la tasa de éxito por intento de forrajeo no cambió con el aumento del número de gaviotas. R. tridactyla presentó una mayor probabilidad de evitar las bandadas con mayor número de gaviotas. Las gaviotas cleptoparasitaron exitosamente menos del 1% de los peces capturados por R. tridactyla. Nuestros resultados sugieren que la interferencia pasiva puede ser costosa para las aves más pequeñas que forrajean en bandadas multiespecíficas.

John M. Maniscalco, William D. Ostrand, Robert M. Suryan, and David B. Irons "PASSIVE INTERFERENCE COMPETITION BY GLAUCOUS-WINGED GULLS ON BLACK-LEGGED KITTIWAKES: A COST OF FEEDING IN FLOCKS," The Condor 103(3), 616-619, (1 August 2001).[0616:PICBGW]2.0.CO;2
Received: 1 July 2000; Accepted: 1 March 2001; Published: 1 August 2001
Black-legged Kittiwake
Glaucous-winged Gull
multispecies feeding flocks
passive interference competition
Prince William Sound
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