The grey fantail (Rhipidura fuliginosa)in New Zealand displays a striking plumage polymorphism. Some individuals are coloured almost entirely black (the "black morph"), while other individuals sport a contrasting brown and white plumage (the "pied morph"). The adaptive significance of plumage polymorphism in this species is unknown. We mapped the relative distribution and frequency of each morph across the entire South Island range of the fantail, and correlated the frequency of the morphs with a variety of ecological variables. The black morph comprised <5% of individuals across the South Island and, contrary to previous observations, was least frequent at the southern extremes of its range. From historical records, the frequency of the black morph also appears to have declined, although we cannot rule out a bias in reporting rates of the black morph in the literature. The relative frequency of the two morphs was not related to vegetation type, annual rainfall, altitude, or mean annual temperature. Although we could not identify an environmental variable that might explain the distribution of the two morphs over the South Island, changes in the relative abundance of each morph suggest a dynamic process that warrants further long-term study.

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