This species was recognized in the late 1990's when the American Ornithologists' Union ( American Ornithologists' Union 1997 ) split the Solitary Vireo complex into 3 species: the Plumbeous Vireo of the western interior, Cassin's Vireo (V. cassinii) of the West Coast, and the Blue-headed Vireo (V. solitarius) of eastern North America. This split was based on molecular genetic studies by Johnson ( Johnson 1995a ) and Murray et al. ( Murray et al. 1994a ), that revealed significant divergence in mitochondrial DNA and allozymes within the complex. The 3 species differ sufficiently in plumage to have been illustrated separately in field guides before the Solitary Vireo complex was split, but field identification of the 3 species requires caution and is not always possible.
Migratory in the U.S. but resident to the south (Figure 1), the Plumbeous Vireo typically breeds in warm, dry, often montane forests of pine, oak and juniper, although populations can be found nesting locally in deciduous woodlands along rivers of the Great Basin.
The Plumbeous Vireo is one of the less studied vireo species, and much confusion surrounds its subspecific taxonomy; biochemical work on this topic remains to be done. The expansion of this species' range in California, where it is now locally sympatric with Cassin's Vireo, has been well documented ( Johnson and Garrett 1974 , Johnson 1994d ), and it is hoped that its new status will focus more research attention on the Plumbeous Vireo.