Transfer experiments of Seychelles warblers to new islands: changes in dispersal and helping behaviour

Young of the cooperatively breeding Seychelles warbler, Acrocephalus sechellensis, frequently remain on their natal territories and act as helpers. Independent reproduction may be constrained by habitat saturation (a shortage of breeding territories) or, alternatively, individuals on good territories may achieve higher fitness by staying and helping than by dispersing and breeding on inferior territories. Transfers of warblers to unoccupied islands showed that both habitat saturation and variation in territory quality dramatically affected the frequency of delayed dispersal. At first there was no cooperative breeding, but as all high-quality areas became occupied, young birds hatched on them began to stay as helpers, rather than occupy breeding vacancies on lower quality territories. However, as the number of helpers on high-quality territories increased, thereby reducing the helper’s indirect component of inclusive fitness benefits from staying at home, it paid some helpers to leave, even to poor territories. Thereafter, young reared on poor territories did better to leave to breed on poor territories, rather than stay at home.