Cassin's Vireo (Vireo cassinii) [XC309474]
by Melissa Hafting from Cowichan Valley B, Cowichan Valley, British Columbia, Canada (song)
Cassin's Vireo (Vireo cassinii) [XC570353]
by Ned Bohman from Sierra Nevada (near Markleeville), Alpine County, California, United States (song)
Subspecies
Probably forms a superspecies with Plumbeous Vireo (Vireo plumbeus) and Blue-headed Vireo (Vireo solitarius) and the three were formerly treated as conspecific, but molecular-genetic studies indicate that they merit treatment as separate species. Breeding range in western Canada overlaps slightly with that of Blue-headed Vireo (Vireo solitarius), and some interbreeding may occur. However, cytochrome b sequence studies indicate as much divergence between the two as between Blue-headed Vireo (Vireo solitarius) and Yellow-throated Vireo (Vireo flavifrons).
The following 2 subspecies are recognised:
cassinii Xántus de Vesey, 1858 - Breeds south-western Canada (southern British Columbia southern from c. 53º north) and western USA (Washington and western Montana south to southern California), disjunctly also in north-western Mexico (mountains of northern Baja California); migrates to western Mexico (south to Oaxaca).
lucasanus Brewster, 1891 - Southern Baja California (western Mexico).
BirdLife International Species Factsheet