Young Seychelles warblers Acrocephalus sechellensis often remain in their natal territories as helpers. Helpers on low-quality territories (as measured by food availability) reduce their parents’ reproductive success, whereas 1-2 helpers on high-quality territories increase their parents’ reproductive success, thereby enhancing their inclusive fitness, in addition to gaining experience(1,2), and opportunities for co-breeding(3). Helpers are mostly females, and we have previously suggested that parents may adjust the sex of their single egg to territory quality(4). We therefore took blood samples from nestlings, and determined sex using random amplified polymorphic DNA (RAPD) markers. We show that biased hatching sex ratios are caused by biased production and not by differential embryo mortality. Unhelped breeding pairs on low-quality territories produce 77% sons, whereas unhelped pairs on high-quality territories produce 13% sons. Breeding pairs that were transferred from low- to high-quality territories switched from the production of male to female eggs. Breeding pairs occupying high-quality territories switched from producing female eggs when no or one helper was present, to producing male eggs when two helpers were present in the territory.