IMPORTANCE OF HABITAT SATURATION AND TERRITORY QUALITY FOR EVOLUTION OF COOPERATIVE BREEDING IN THE SEYCHELLES WARBLER

COOPERATIVE breeding, which often involves young remaining on their natal territory and helping their parents to raise subsequent broods1-3, is mostly explained by habitat saturation: young are constrained from becoming independent breeders by a shortage of breeding territories2,4. Here I present two lines of evidence against this hypothesis for the cooperatively breeding Seychelles warbler Acrocephalus sechellensis. First, territory quality has a significant effect on dispersal: vacancies arising on territories are mostly filled by prebreeding birds from territories of the same or lower quality. Second, individuals that delay reproduction in high-quality territories, but which eventually breed there, have greater lifetime fitness than those that disperse at one year of age and breed immediately in lower-quality territories. These results support the 'benefits of philopatry,5,6 hypothesis, which emphasizes the lifetime inclusive fitness benefits from staying at home. The transfers of warblers to unoccupied islands was the strictest experimental test of this hypothesis. At first there was no cooperative breeding, but as all high-quality areas became occupied, young birds born on high-quality territories began to stay as helpers, rather than occupying breeding vacancies on low-quality territories. Therefore habitat saturation and territory quality are both involved in the evolution of cooperative breeding.