Avian sex determination is chromosomal; however, the underlying mechanisms are not yet understood. There is no conclusive evidence for either of two proposed mechanisms: a dominant genetic switch or a dosage mechanism. No dominant sex–determining gene on the female–specific W chromosome has been found. Birds lack inactivation of one of the Z chromosomes in males, but seem to compensate for a double dose of Z–linked genes by other mechanisms. Recent studies showing female–specific expression of two genes may support an active role of the W chromosome. To resolve the question of avian sex determination the investigation of birds with a 2A : ZZW or 2A : Z0 genotype would be decisive. Here, we report the case of an apparent 2A : ZZW great reed warbler (Acrocephalus arundinaceus) female breeding in a natural population, which was detected using Z–linked microsatellites. Our data strongly suggest a role of W–linked genes in avian sex determination.