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The endemic Tahiti reed-warbler Acrocephalus caffer occurs in two distinct morphs, a typical or ‘yellow’ morph and a melanic or ‘dark’ morph, which are found together in the valleys of the eastern and central parts of the island of Tahiti (Society Islands, French Polynesia). We investigated the molecular basis of the plumage colour polymorphism in this species using sequences of the melanocortin-1 receptor (MC1R), a gene often found associated to melanism in birds. We found that the MC1R genotype was perfectly associated with plumage colour in the Tahiti reed-warbler, with the same nonsynonymous substitution that showed a correlation with phenotype in the Caribbean bananaquit Coereba flaveola. An heterozygous reed-warbler at this site presented a melanic phenotype, suggesting that the melanic allele is dominant. All other Polynesian reed-warbler species, which do not have a melanic morph, shared the ‘yellow’ nucleotide at this position. These results suggested that the same mutation point was linked to a melanic polymorphism in two unrelated passerine birds.