FACTORS AFFECTING KLEPTOPARASITISM AND PREDATION RATES UPON A COLONY OF AUDOUINS GULL (LARUS-AUDOUINII) BY YELLOW-LEGGED GULLS (LARUS-CACHINNANS) IN SPAIN

We studied part of the world's largest breeding colony of Audouin's Gull (Larus audouinii) in the Ebro Delta, NE Spain, in 1992, to evaluate predation and kleptoparasitism effects by Yellow-legged Gulls (Larus cachinnans). Kleptoparasitic attacks were the most frequent kind of disturbance, concentrated during the period when Yellow-legged Gull chicks were being raised. This also coincided with a moratorium period for commercial fisheries when the food resource provided by fishing vessels was unavailable. More attacks occurred either early or late in the day when the Audouin's Gulls returned to the colony from feeding sites. The success rate was higher for long chases and for large parasitic groups of gulls. Subadult and adult Yellow-legged Gulls had similar kleptoparasitic success. Yellow-legged Gull predation on Audouin's Gull eggs and chicks was low and appeared not to affect the breeding performance of Audouin's Gull.