Molecular evidence that the Channel Islands populations of the orange-crowned warbler (; Aves: Passeriformes: Parulidae) represent a distinct evolutionary lineage.

We used molecular data to assess the degree of genetic divergence across the breeding range of the orange-crowned warbler () in western North America with particular focus on characterizing the divergence between populations on the mainland of southern California and on the Channel Islands. We obtained sequences of the mitochondrial gene and genotypes at ten microsatellite data for 192 from populations spanning all four recognized subspecies. We recovered shallow, but significant, levels of divergence among populations across the species range. Our results suggest that island isolation, subspecies (delineation by morphology, ecological, and life-history characteristics), and isolation-by-distance, in that order, are the variables that best explain the geographic structure detected across the range of . Populations on the Channel Islands were genetically divergent from those on the mainland. We found evidence for greater gene flow from the Channel Islands population to mainland southern California than from the mainland to the islands. We discuss these data in the context of differentiation in phenotypic and ecological characters.