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Host choice and success of gulls and terns kleptoparasitizing brown pelicans

Abstract Little agreement has been reached as to whether kleptoparasitic gulls,Larusspp., preferentially target adult, juvenile or just successful brown pelicans,Pelecanus occidentalis, and no studies have determined kleptoparasite success when targeting pelicans of different age classes. In 1991 and 1992, foraging brown pelicans and kleptoparasitic laughing gulls,L. atricilla, roseate terns,Sterna dougallii, and sandwich terns,S. sandvicensis, were studied to determine host choice, kleptoparasite success and exploitability of pelicans by gulls and terns. Adult pelicans had a higher foraging success than juveniles in 1991 but not in 1992. Gulls and terns were more likely to target adult pelicans than juveniles, but the strength of this effect was stronger in 1991 than in 1992. Gulls and terns were also more likely to target successful than unsuccessful pelicans, and the strength of this effect did not differ between years. Kleptoparasites had a higher probability of success when targeting adult rather than juvenile pelicans and when targeting successful rather than unsuccessful pelicans. Successful pelicans were more likely than unsuccessful pelicans to use evasive manoeuvres, and prey capture success among kleptoparasites was greater when successful pelicans used evasive manoeuvres. Handling times did not differ between the two pelican age classes. Kleptoparasitic gulls and terns may not necessarily differentiate between ‘reliable’ and ‘exploitable’ victims, as previously suggested, because in the present study adult pelicans were the more reliable class, as well as the more exploitable class. 1997 The Association for the Study of Animal Behaviour