This study describes qualitatively and quantitatively the kleptoparasitic behavior of the Brown-hooded Gull (Larus maculipennis) and Grey-hooded Gull (Larus cirrocephalus) on the American Oystercatcher (Haematopus palliatus), and considers the influence of environmental variables on the occurrence and success of the kleptoparasitism. Focal and scan samplings were performed, recording a total of 358 kleptoparasitic attempts. The overall occurrence rate was 1.2 ± 1.3 attempts per 5 min., of which 42% of attempts were successful. All kleptoparasitic attempts were performed when oystercatchers were feeding on Stout Razor Clams (Tagellus plebeius). Gulls stole food from Oystercatchers by two kleptoparasitic tactics; running (used in 40% of cases) and flying (used in 60% of cases). A significant difference in the rate of success of kleptoparasitism and an increase in the use of flying kleptoparasitism were observed under windy conditions. Gulls showed limited ability to open clams by themselves, and never swallowed whole clams. Kleptoparasitic attacks occurred within three seconds of the clam being ingested by the host, indicating the accurate kleptoparasitic skills of hooded gulls. Possible factors that affected the decisions taken by gulls about when and how to start the robbing behavior are discussed. Features of the kleptoparasitic behavior performed by hooded gulls on oystercatchers provide some relevant questions regarding the “generalist” or “specialist” character of these parasites.
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Vol. 25 • No. 2