Many avian species from the temperate zone show low levels of genetic diversity when compared with species from the tropics, although there are some interesting exceptions, and species whose ranges occur across the temperate–tropical boundary could have characteristics of either or both zones. The distribution of the Streak-backed Oriole (Icterus pustulatus) extends from northwestern Sonora in Mexico south along the Pacific coast to Costa Rica. In addition, there is a population on the Tres Marías Islands. Northern populations are migratory and dichromatic (sexes different), whereas southern populations are nonmigratory and monochromatic (sexes alike). We sequenced mtDNA (control region domain I) from 102 individuals across the range of this species. Maximum parsimony and maximum likelihood analyses did not support the existence of multiple species within Icterus pustulatus based on mtDNA. However, populations from the Tres Marías Islands seem to have been isolated long enough to have fixed differences from populations on the mainland. The species complex shows an uncommon phylogeographic pattern, with shallow haplotype trees and weak geographic structure, suggesting that it has undergone recent population expansion. We recommend that the insular populations be considered a separate species, Icterus graysonii, given the fixed differences from mainland birds in multiple characteristics, including mtDNA and plumage patterns.