12- Seychelles warblers:Complexities of the helping paradox

IntroductionThe Seychelles warbler (Acrocephalus sechellensis), a passerine endemic to the Seychelles archipelago, is a facultatively cooperative breeder that lives either in pairs or small groups. Breeding groups normally consist of a dominant pair and one to three subordinates, although up to nine have been observed. Subordinates can be of either sex, and are often offspring that have delayed dispersal and remained in their natal territory (Komdeur 1992; Richardson et al. 2002; Eikenaar et al. 2008, 2010). By the 1960s the last remnant population of this then critically endangered species was confined to Cousin Island (Figure 12.1). Subsequent conservation actions, including the restoration of forest habitat and the establishment of new populations through translocations, have provided unique opportunities to study the evolutionary ecology of cooperative breeding in this species. We have followed the entire world population of the Seychelles warbler since our study started in 1981. In 1997, however, we increased the intensity of work on Cousin and since then nearly all individuals, including fledglings, have been captured, ringed, blood sampled, sexed, and monitored for breeding and status. Molecular tools have been used to assign the sex and genetic parentage of young birds and to determine levels of relatedness between individuals. In addition, a comprehensive set of behavioral, life history, and annual fitness parameters have been recorded for nearly all individuals, providing important opportunities for assessing changes in social behavior. The lack of interisland dispersal (Komdeur et al. 2004a), combined with sampling of the entire population, provides a rare opportunity to monitor the survival, reproduction, and lifetime fitness of all individuals within the population. Our long-term research program into cooperative breeding, hand-in-hand with conservation actions, has created an experimental system in which we can attempt to unravel the factors that alter the cost-benefit trade-offs of cooperative breeding. Over time, this system has become proof of the power of the experimental methods and of the corrective value of long-term studies, where iterative examination with longer-term data sheds new insights on short-term findings. Here we detail various findings that have allowed us to uncover how changing social and ecological factors influence the form and function of reproductive competition and cooperation in the Seychelles warbler. We also outline how molecular genetic tools have given us a better understanding of the species’ cooperative breeding system and the selective factors that favor switching between different forms of cooperative and independent breeding.