KLEPTOPARASITISM BY KERMADEC PETRELS, JAEGERS, AND SKUAS IN THE EASTERN TROPICAL PACIFIC - EVIDENCE OF MIMICRY BY 2 SPECIES OF PTERODROMA

We studied the ecology and behavior of pelagic seabirds in the Eastern Tropical Pacific (1984-1992). We hypothesize that the absence of klep toparasitism (hereafter ''parasitism'') by jaegers and skuas (subfamil y Stercorariinae, hereafter ''skuas'') on Kermadec Petrels (Pterodroma neglecta) and Herald Petrels (P. arminjoniana) observed in this study , compared to regular attack on procellariids of similar size, resulte d from Batesian mimicry by the former of skuas. As mimics of skuas, Ke rmadec and Herald petrels avoided being kleptoparasitized because skua s do not parasitize conspecifics. We also document regular parasitism by Kermadec Petrels on other large procellariids, and further hypothes ize that this petrel is successful as a parasite because it is a forag ing mimic of the subadults of the larger skuas (Pomarine Jaegers [Ster corarius pomarinus] and South Polar Skuas [Catharacta maccormicki]) th rough its similarity of color pattern, flight profile, and behavior wh en initiating an attack, and because the large skuas (its models) are very effective as parasites. This petrel's incidence of attack and fre quent use of alternate feeding methods suggests that it is a less spec ialized parasite than are the skuas. Kermadec Petrels prefer the same hosts and use a similar method of attack as do the large skuas, which achieve a very high rate of success in the Eastern Tropical Pacific be cause of their ability to threaten hosts through size-mediated aggress iveness. The Kermadec Petrel is smaller or similar in size to its pref erred hosts and is not as aggressive as large skuas, but its rate of s uccess as a parasite is higher than expected.