Young Seychelles warblers (Acrocephalus sechellensis) frequently remain in their natal territories as helpers. Helpers on low-quality territories (as measured by food availability) reduce their parents' future reproductive success, whereas helpers on high-quality territories increase their parents' future reproductive success, thereby improving their own indirect component of inclusive fitness. Helpers are mainly females, which remain longer in their natal territories than males. The Seychelles warbler shows extreme skews in sex ratios of offspring at six months of age, varying from mainly males on low-quality territories to mainly females on high-quality territories. It appears that breeding birds avoid having helpers on low-quality territories and gain helpers on high-quality territories, thereby increasing their reproductive success. There is evidence that the biased sex ratio is caused by biased production, and not because of biased mortality at younger ages: (i) cumulative mortality from the egg stage to the stage at six months of age is insufficient to generate a consistent deviation from sex ratio parity; (ii) all 30 nestlings produced by two pairs on low-quality territories survived to be sexed as sons, and all nestlings produced by two pairs on high-quality territories survived to be sexed as 17 daughters and one son; and (iii) in addition, breeding pairs that were transferred from low- to high-quality territories, switched from the production of male to female nestlings.