HOST CHOICE AND SUCCESS OF GULLS AND TERNS KLEPTOPARASITIZING BROWN PELICANS

Little agreement has been reached as to whether kleptoparasitic gulfs, Larus spp., preferentially target adult, juvenile or just successful brown pelicans, Pelecanus occidentalis, and no studies have determined kleptoparasite success when targeting pelicans of different age class es. In 1991 and 1992, foraging brown pelicans and kleptoparasitic laug hing gulls, L. atricilla, roseate terns, Sterna dougallii, and sandwic h terns, S. sandvicensis, were studied to determine host choice, klept oparasite success and exploitability of pelicans by gulls and terns. A dult pelicans had a higher foraging success than juveniles in 1991 but not in 1992. Gulls and terns were more likely to target adult pelican s than juveniles, but the strength of this effect was stronger in 1991 than in 1992. Gulls and terns were also more likely to target success ful than unsuccessful pelicans, and the strength of this effect did no t differ between years. Kleptoparasites had a higher probability of su ccess when targeting adult rather than juvenile pelicans and when targ eting successful rather than unsuccessful pelicans. Successful pelican s were more likely than unsuccessful pelicans to use evasive manoeuvre s, and prey capture success among kleptoparasites was greater when suc cessful pelicans used evasive manoeuvres. Handling times did not diffe r 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 exploitabl e class. (C) 1997 The Association for the Study of Animal Behaviour.