RICHARDSON F, JURY L, BLAAKMEER K, KOMDEUR J & BURKE T (2001) Themadag 'Evolutie van Sociaal Gedraag': Unravelling the complicated sex life of the Seychelles warbler. LIMOSA 74 (2): 74-75.

The cooperatively breeding Seychelles Warbler Acrocephalus sechellensis is a rare endemic of the Seychelles islands. By 1959, human disturbance had pushed this species to the verge of extinction and only 26 individuals remained confined to the island of Cousin. As a result of long-term management the population has since recovered and has been the focus of study since 1985. Accurate parentage analysis is now required to address questions relating to the genetic benefits of mate choice, inbreeding avoidance and the evolution of cooperative breeding. In 1999 at least 97% of the individuals (N = 293 adults, 104 territories) were marked and monitored on Cousin Island. We developed a series of polymorphic microsatellite markers so that we could assign parentage to the 1-2 (or rarely 3) eggs produced in each nesting attempt. We confirmed the maternity of an adult female in a sample of 45 territories. The primary female was the mother for 84% of offspring, a helper female of 16%, and mixed maternity was detected in three out of five occasions in which it could have occurred (nests with multiple offspring and multiple attendant females). Helper females therefore gain both direct benefits by co-breeding and indirect fitness benefits by helping to raise siblings. We also found that 36% of 55 chicks resulted from extra-group paternity. We were able to assign all these chicks to extra-pair males, which were always breeding males, normally from a neighbouring territory. The high rate of extra-group paternity contrasts with the previously consistent rarity of extra- pair paternity in populations of passerine birds on islands. In a preliminary analysis, we found that extra-pair paternity was more likely when the territorial male was related to the breeding female, which can arise when an offspring remains as a helper and subsequently inherits a territory, Extrapair mating may therefore be an adaptive strategy to avoid inbreeding in this cooperative system.

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