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

In cooperatively breeding species, helping at the nest and budding off part of the natal territory have been advanced as strategies to increase fitness in an environment that is saturated with territories. The importance of helping or territory budding as a determinant of lifetime reproductive success (LRS) has been debated because the potential benefits of both strategies could not be separated. Here we test the causes and the immediate and future fitness consequences of single dispersal decisions taken by male Seychelles warblers (Acrocephalus sechellensis). Males breeding in high-quality territories (high food abundance) have significantly higher LRS than similar-aged males budding off part of the parental territory. Initially, budders have a low reproductive success (because of limited food resources or absence of a breeding partner). However, they have a long life span and inherit high-quality territories through site dominance, by which they gain higher LRS than breeders on low-quality territories, helpers, or floaters. Experimental creation of male breeding territory vacancies showed that most young males became budders because of intense competition for high-quality territories. The translocation of warblers to the previously unoccupied Aride Island shows that males behave according to the expected fitness benefits of each dispersal strategy. In the absence of competition for territories on Aride, all young males bred in high-quality territories. However, after saturation of high-quality habitat with territories, most males became budders rather than breeders on low-quality habitat, helpers, or floaters.