Trios of brown skuas at Bird Island, South Georgia: incidence and composition

Cooperative breeding is unusual in seabirds, but recorded in varying proportions in Brown Skuas (Stercorarius [= Catharacta] lonnbergi). With no evidence for kin selection or delayed dispersal, nor for any reproductive advantage. the reason for its occurrence in skuas is uncertain. This study describe, the incidence, composition, and breeding characteristics of trios of Brown Skuas at Bird Island. South Georgia. Trios were recorded in nine seasons, including a polygynous and a polyandrous trio found in both 2000 2001 and 2001-2002. In some years, birds hatched three chicks from supranormal clutches, which is exceptional in skuas and jaegers, yet their overall performance was no better than monogamous pairs. I suggest that compared with closely related taxa, Brown Skuas have a slight behavioral or genetic predisposition that facilitates cooperative breeding. Cooperation conveys a minor advantage in territory acquisition, but none in terms of reproductive success (particularly for polygynous groups). and consequently has not proliferated at most colonies.