The Black-throated Gray Warbler (Setophaga nigrescens) is a migratory songbird with a widespread and continuous distribution in the North American West. Previously published morphological data support the designation of 2 putative subspecies based on subtle differences in plumage and song; however, whether these patterns are clinal or discrete has yet to be determined. Furthermore, given that patterns of morphological variation occur across multiple distinct habitats and geographical boundaries, underlying population genetic structure may be present. To investigate this question and explore the evolutionary history of S. nigrescens, we conducted the first range-wide phylogeographic analysis of this species using genetic (mitochondrial DNA [mtDNA], ND2 gene sequences), morphological (body measurements), and environmental (niche modeling) data. Overall, our results identified 2 major mtDNA clades. One clade occurs east of the Cascade-Sierra ranges throughout the southwestern, Great Basin, and Rocky Mountain regions in North America (Interior clade), while a second widespread clade is comprised of birds that span the Pacific Northwest, and also across the Great Basin and southwest into the Rocky Mountains (Coastal clade). Divergence time estimates suggest a Pleistocene epoch vicariance event. Environmental niche models predict habitat modification and fragmentation from glacial–interglacial cycles, which were frequent over the Pleistocene. Range expansion northwards following the Last Glacial Maximum was identified from neutrality tests and shifts in suitable habitat from the Mid-Holocene to present-day conditions, where both clades are likely experiencing secondary contact. We found support for nonequivalent environmental niches among both clades based on a spatial analysis of environmental variables, with the Coastal clade showing greater tolerance to both temperate and xeric climatic conditions across the North American West; however, the environmental space occupied by both clades was more similar to each other than expected by chance. High levels of geographic overlap and a lack of discrete patterns in morphology suggest few contemporary reproductive barriers to gene flow.

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