Male Seychelles warblers use territory budding to maximize lifetime fitness in a saturated environment

In cooperatively breeding species, helping at the nest and buddingoff part of the natal territory have been advanced as strategiesto increase fitness in an environment that is saturated withterritories. The importance of helping or territory buddingas a determinant of lifetime reproductive success (LRS) hasbeen debated because the potential benefits of both strategiescould not be separated. Here we test the causes and the immediateand future fitness consequences of single dispersal decisionstaken by male Seychelles warblers (Acrocephalus sechellensis).Males breeding in high-quality territories (high food abundance)have significantly higher LRS than similar-aged males buddingoff part of the parental territory. Initially, budders havea low reproductive success (because of limited food resourcesor absence of a breeding partner). However, they have a longlife span and inherit high-quality territories through sitedominance, by which they gain higher LRS than breeders on low-qualityterritories, helpers, or floaters. Experimental creation ofmale breeding territory vacancies showed that most young malesbecame budders because of intense competition for high-qualityterritories. The translocation of warblers to the previouslyunoccupied Aride Island shows that males behave according tothe expected fitness benefits of each dispersal strategy. Inthe absence of competition for territories on Aride, all youngmales bred in high-quality territories. However, after saturationof high-quality habitat with territories, most males becamebudders rather than breeders on low-quality habitat, helpers,or floaters.