Aptly described as “the spirit of the live oak tree” ( Van Fleet 1919 ), Hutton's Vireo is a year-round resident in mixed evergreen forests and woodlands of western North America, and can be particularly common in areas where live (evergreen) oaks predominate. This species occurs throughout most of its breeding range during every month of the year, and often is described as the only non-migratory vireo in the United States. Yet the southwestern population is partly migratory and others exhibit conspicuous, though largely local, seasonal movements.
Unobtrusive, easily overlooked, and little-studied, this vireo is noticed most often by its persistent but insipid two-parted song. Individuals begin singing in late winter and begin nesting in early spring. Hence, they are sometimes under-represented on breeding bird surveys and atlas projects, which usually begin after the peak singing period for this species. The sexes are alike in plumage, are at least seasonally monogamous, and both participate in nest-building, incubation, and caring for the young. In winter, they often join mixed-species flocks which also typically contain Ruby-crowned Kinglets (Regulus calendula). Hutton's Vireos are similar enough in plumage and behavior to the more common and widespread Ruby-crowned Kinglet that their identification can be a challenge, especially for those unfamiliar with western birds.
At the persistent request of colleague Spencer F. Baird, John Cassin reluctantly named this species after William Hutton, a young naturalist whom he did not know but who collected the type specimens in 1847 in Monterey, CA ( Mearns and Mearns 1992a ).
Up to twelve subspecies have been described based on variation in size and plumage ( Phillips 1991 ). Coastal and interior forms exist in disjunct allopatry where populations are separated by wide desert. Preliminary biochemical work suggests that these forms are so different genetically that they may warrant full species status ( Cicero and Johnson 1992 ).