Aptly described as “the spirit of the live oak tree” (1), Hutton’s Vireo is a year-round resident in mixed-evergreen forests and woodlands from southwestern British Columbia to Guatemala and can be particularly common in areas where live (evergreen) oaks predominate. Throughout most of its breeding range, the Hutton's Vireo occurs during every month of the year and often is described as the only non-migratory vireo in the United States. Yet the southwestern interior population is partly migratory and other populations exhibit conspicuous, though largely local, seasonal movements.
Unobtrusive, easily overlooked, and little-studied, the Hutton's 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, the species can be 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. During the overwintering period, the species often joins mixed-species flocks which also typically contain Ruby-crowned Kinglet (Regulus calendula). Hutton’s Vireo is similar enough in plumage and behavior to the more common and widespread Ruby-crowned Kinglet that identification can be difficult for those unfamiliar with the species.
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, California (2).
Based on variation in size and plumage, 12 subspecies have been described (3), in two groups: Pacific (huttoni Group) and interior (stephensi Group). Preliminary biochemical work on V. h. huttoni and V. h. stephensi suggested that these subspecies, which are separated by a broad expanse of desert, are so different genetically that they may warrant full species status (4).