Kleptoparasitism By Kermadec Petrels, Jaegers, And Skuas In The Eastern Tropical Pacific: Evidence Of Mimicry By Two 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 kleptoparasitism (hereafter "parasitism") by jaegers and skuas (subfamily 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, resulted from Batesian mimicry by the former of skuas. As mimics of skuas, Kermadec and Herald petrels avoided being kleptoparasitized because skuas do not parasitize conspecifics. We also document regular parasitism by Kermadec Petrels on other large procellariids, and further hypothesize that this petrel is successful as a parasite because it is a foraging mimic of the subadults of the larger skuas (Pomarine Jaegers [Stercorarius pomarinus] and South Polar Skuas [Catharacta maccormicki]) through its similarity of color pattern, flight profile, and behavior when initiating an attack, and because the large skuas (its models) are very effective as parasites. This petrel's incidence of attack and frequent use of alternate feeding methods suggests that it is a less specialized 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 because of their ability to threaten hosts through size-mediated aggressiveness. The Kermadec Petrel is smaller or similar in size to its preferred hosts and is not as aggressive as large skuas, but its rate of success as a parasite is higher than expected. Received 1 September 1991, accepted 22 June 1992. KLEPTOPARASITISM (hereafter termed "parasitism") is a well-known feeding method of seabirds, particularly for skuas (Catharacta spp.) and jaegers (Stercorarius spp.; subfamily Stercorariinae, hereafter referred to collectively as "skuas"), frigatebirds (Fregata spp.), gulls (Larus spp.) and, to a lesser extent, terns (Sterna spp.), boobies (Sula spp.), pelicans (Pelecanus spp.), cormorants (Phalacrocorax spp.), and members of the Procellariiformes (reviewed by Furness 1987a, b, Duffy 1980, 1982, 1989). In procellariiforms, the use of parasitism as a regular means of feeding has been described only in the Waved Albatross (Diomedea irrorata; Duffy 1980), and no procellariiform parasite has been known to tenaciously pursue hosts as do skuas, the most highly evolved of avian parasites. Herein, we describe skualike parasitism by a procellariid, the Kermadec Petrel (Pterodroma neglecta). Parasitism by Parasitic Jaegers (S. parasiticus), Great Skuas (C. skua), and South Polar Skuas (C. maccormicki) has been studied extensively on the breeding grounds (reviewed in Furness 1987a, b) and at a coastal estuary on the migratory route (Parasitic Jaeger; Taylor 1979). However, few studies have documented the foraging behavior of skuas away from the breeding grounds, other than species parasitized at given locations (but see Duffy 1980, Sinclair 1980, Furness 1983). Skuas possess a distinct and highly conspicuous white patch in the inner primary region of their otherwise dark-colored wings (Harrison 1987, Olsen 1989). In flight, this mark can be seen at a great distance at any angle except when the birds are flying directly toward, or away from, the observer. As a result of this distinctive feature, and the tenacious, aggressive, and highly skilled qualities of skuas as parasites, they qualify as aposematic species. Aposematism is a condition used by more dangerous/ noxious species to advertise their particular qualities and, thus, ensure the desired response from animals with whom they interact (usually avoidance by predators; reviewed in Pough 1988, Malcolm 1990). In skuas, aposematism may serve as an affective, long-distance signal between conspecifics, which do not parasitize one another (Furness 1987a), thereby preventing lessrewarding interactions between individuals of equal ability, or between kin (see Malcolm 1990). Aposematism might also benefit skuas as a