Mitochondrial DNA Variation and the Taxonomic Status of the Large-Billed Savannah Sparrow

The Savannah Sparrow (Passerculus sandwichensis ) is widely distributed in North America. The species includes several phenotypically distinct and geographically restricted forms, some of which were once considered species, as well as a more "typical" or familiar form that is found throughout much of the continent. We assessed mitochondrial DNA (mtDNA) differentiation between one of the divergent forms, the large-billed Savannah Sparrow (P. s. rostratus ), and representatives of more "typical" forms of the species. Although van Rossem (1947) proposed that Savannah Sparrow morphology changed gradually from typical (interior North American forms) to large-billed forms (coastal Mexican forms), we suggest that the large-billed Savannah Sparrow is a distinct taxon and not merely the end-point of a gradual cline.