Kleptoparasitism by Brown Skuas on albatrosses and giant-petrels in the Indian Ocean

Auk, The, Apr 1999 by Spear, Larry B, Howell, Steve N G, Oedekoven, Cornelia S, Legay, Delphine, Bried, Joel

The form of Brown Skua breeding on the Crozet and Kerguelen archipelagos is the largest, most tenacious Catharacta (Furness 1987a, Mailing-Olsen and Larsson 1997), and conceivably it could succeed at kleptoparasitizing the less-mobile giant-petrel and "great" albatross. An analysis of the expected success rate of skuas as a function of the parasitehost mass differential (Fig. 1), however, revealed a significant negative relationship, indicating that, on average, the success rate of kleptoparasites should become zero as the mass of the host approaches 1.4 times that of kleptoparasite. Thus, it is unlikely that the behavior of the skuas that attacked the Northern Giant-Petrel and Wandering Albatross was adaptive. We suspect that because the two skuas were young and less experienced, they overestimated their capabilities.

The greatest deviation from the predicted success rate shown in Figure 1 was the relationship reported by Furness (1978), in which Great Skuas (Catharacta skua) achieved a much higher success rate than expected (success rate = 0.30 from hosts 2.2 times larger) when attacking Northern Gannets returning to a breeding colony. In his study, gannets were regular victims of skuas. The higher success rate occurred mainly because (1) several skuas often attacked a single gannet simultaneously, and (2) the gannets fed locally in waters that were highly productive. As a result, the gannets could find additional food relatively easily so that the occasional loss of food to skuas was of low cost. In this context, the relationships among Brown Skuas and albatrosses may be different. Albatrosses are probably less likely to be attacked by groups of Brown Skuas because colonies of the latter tend to be small (Stahl et al. 1996) compared with those of Great Skuas (Klomp and Furness 1992). Furthermore, the duration of foraging trips of albatrosses is much longer than that of other seabirds (Weimerskirch et al. 1988, Jouventin and Weimerskirch 1990, Prince et al. 1992). Therefore, giving up food to skuas is of higher cost, and albatrosses would probably be less likely to do so compared with Northern Gannets. These differences might help to explain the paucity of reports of attempted kleptoparasitism by skuas on large procellarforms.

Acknowledgments.-We thank the officers and crew of R/V Marion Dufresne for cooperation and great food. We are especially grateful to Pierre Jouventin for inviting us. We thank David Ainley, Lisa Ballance, Stephen Maxson, Jeff Marks, and an anonymous reviewer for helpful comments on the manuscript. Funding to LBS, SNGH, and CSO was by the U.S. National Science Foundation supplement to grant OPP95-26865 to David Ainley and LBS; support to JB and DL was provided by the Institut Francais pour la Recherche et al Technologie Polaires.

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