John Kirk Townsend first described and named this warbler in 1837 from a pair collected at Fort Vancouver (now Vancouver, WA) near the Columbia River. The species was named for its apparently solitary and secretive behavior, foraging mainly in the interior and canopy of tall coniferous forests. Although it is regularly heard singing, this species is often difficult to see.
The Hermit Warbler breeds in coniferous forests of the Coast, Cascade, and Sierra Nevada mountain ranges of southern Washington, Oregon, and central and northern California. It usually builds its nest saddled on limbs well above ground and concealed by branches. Most individuals migrate along the Coast, Cascade, and Sierra Nevada mountain ranges, wintering in the pine-oak (Pinus-Quercus) and pine forests of the mountains of Mexico and Guatemala, where it forages in mixed-species flocks.
The Hermit Warbler is a member of the Black-throated Green Warbler superspecies, which usually includes the Black-throated Green (Setophaga virens), Golden-cheeked (S. chrysoparia), and Townsend's (S. townsendi) warblers. Hermit and Townsend's warblers are probably sister species and apparently diverged in the latter part of the Pleistocene. Currently, where the ranges of these two species overlap in Washington and Oregon, they hybridize extensively along narrow hybrid zones.
The biology of the Hermit Warbler is poorly understood, and much of what is known about the species' ecology, behavior, migration, and breeding is anecdotal. Recently, this species has been studied more extensively with regard to hybridization in Oregon and Washington (Morrison and Hardy 1983a, S. Rohwer and C. Wood pers. comm., SFP).
According to Breeding Bird Survey data, Hermit Warbler population densities appear to have remained stable during the past 30 years, although loss of suitable wintering and breeding habitat has likely decreased this species' overall population size. Winter and breeding populations are considered vulnerable because this species has relatively small populations and a narrow geographic distribution and is a habitat specialist. In addition, analysis of specimens and behavioral studies suggest that the Townsend's Warbler is outcompeting and replacing the Hermit Warbler in significant portions of its range.