Evidence of former allopatry of the two colour phases of Lesser Snow Goose (Chen caerulescens caerulescens).

Plumage color is distributed clinally in the Gulf Coast population of the dimorphic Lesser Snow Goose (Chen caerulescens caerulescens); the white phase predominates in the west, and the blue phase in the east. A similar distribution occurs in the breeding colonies of this population. Historical evidence, stretching back to the mid-18th century, shows that the two phases were almost allopatric until the third decade of the 20th century. Allozyme variants at 6 loci also support the conclusion that the morphs were until recently two distinct taxa. The recent merging of the taxa probably is due to a change in winter feeding habits that allowed birds of both morphs to meet in the rice-growing areas of inland Texas and Louisiana. Because pair formation occurs at this time, this change permitted gene flow to occur between the morphs. There is no evidence of reduced fitness among mixed pairs, and interbreeding among the morphs is common. We know of no other case of a historically documented merging of two formerly allopatric taxa of birds where interbreeding is so widespread and where there is no evidence of reduced fitness of the hybrids. Received 30 January 1987, accepted 26 February 1988.