Named in honor of the nineteenth-century ornithologist, John Cassin, who published the first comprehensive work on the birds of the western United States, Cassin's Vireo is a common member of many coniferous and mixed-forest bird communities of far western North America. Although somewhat drab in appearance, and slow and deliberate in motion, Cassin's Vireo is a conspicuous element of the forest, both in its loud and tireless singing and its raucous scolding call, a “rasping, nerve-grating war-cry” ( Dawson 1923 ), given in defense of nests.
Cassin's Vireo was formerly considered conspecific with the Blue-headed (V. solitarius) and Plumbeous (V. plumbeus) vireos in a group of nine subspecies that were together known as the Solitary Vireo ( American Ornithologists' Union 1957 ). This Solitary Vireo complex was recently split following molecular genetics studies ( Murray et al. 1994a , Johnson 1995a ) that demonstrated differences among these three well-recognized plumage types ( American Ornithologists' Union 1997 ). Currently, the Cassin's Vireo includes two subspecies: the nominate migratory subspecies, V. c. cassinii, which breeds along the west coast of North America, and a non-migratory Mexican subspecies, V. c. lucasanus, found only in the far southern Cape District of Baja California Sur.
Unlike many other vireos and migratory songbird species, Cassin's Vireo appears to be steadily increasing in abundance throughout most of its breeding range, particularly over the past 20 years ( Sauer et al. 1999 ). Although susceptible to cowbird parasitism, this species may benefit from the many large tracts of relatively undeveloped and cowbird-free lands that remain in the western mountains where these birds raise their young. Although a breeder in several states with strong histories of bird research, many basic aspects of this vireo's biology remain poorly studied, perhaps because of its former subspecific status. Its new specific status should help focus research attention on this interesting bird.