The Song Sparrow, Melospiza melodia, as a ring species: patterns of geographic variation, a revision of subspecies, and implications for speciation

Identification and analysis of ring species are important to our understanding of evolution and speciation. We review geographic variation in the Song Sparrow (Melospiza melodia) in the context of a ring species, one of few known from the highly vagile class Aves. Although 52 subspecies have been named, our reassessment of morphological variation across the entire species reveals that 25 subspecies are diagnosable throughout the species' range, which includes much of the North American continent. We include an analysis of plumage variation in M. m. heermanni of coastal California and M. m. follax of the Sonoran Desert and a description of the contact zone between these subspecies, which forms the contact points of a ring centred around the Sierra Nevada and Mojave Desert. These two subspecies come into contact in a narrow hybrid zone, where interbreeding is limited. In addition to substantial differences in plumage, songs of these subspecies vary concomitantly with differences in the structure of occupied habitat. Females tend to exhibit assortative preferences for plumage and song and males exhibit assortative recognition (and associated agonistic behaviour) of song. Plumage variation across the Song Sparrow varies according to climate and habitat, suggesting both geographical and ecological components to the species' diversification.