Inhabiting open coniferous and mixed coniferous-deciduous forests of western North America, Williamson's Sapsucker has been considered a sensitive indicator species because of its specific habitat requirements. Like other sapsuckers, it drills conspicuous rings of holes (“sap wells”) into tree trunks, specializing on coniferous sap and phloem. Breeders switch to a diet of ants during the nestling period.
Unlike all other woodpeckers, Williamson's Sapsucker exhibits spectacular sexual differences in plumage. Males are black with bright red, yellow, and white. Females are mostly cryptic brown with little contrast except for their yellow bellies. These plumage differences confused early naturalists, who thought the two sexes were separate species.
John Cassin first described the species as Picus thyroideus in 1851 from two female specimens collected by John G. Bell in California (Cassin 1852). Cassin (1856) changed the genus provisionally to Melanerpes and first applied the common name Black-breasted Woodpecker. John Newberry (1857) collected a male in 1855 in southern Oregon and named it Williamson's Woodpecker (Picus williamsonii). Both “species” were renamed under the new genus Sphyrapicus by Baird (1857). Two other common names of thyroideus included Baird's (1857) Brown-headed Woodpecker and Cooper's (1870) Round-headed Woodpecker.
The confusion between sexes and species ended when Henry Henshaw verified in 1873 that the two were a single species as he observed a mated pair at a nest in Colorado (Henshaw 1874). The first ornithological compilation to treat both species as one was Coues (1874) using the common name “Black-breasted or Williamson's Woodpecker”. The name sapsucker came into use after being applied to Sphyrapicus in the first checklist of American Ornithologists' Union (1886), which also standardized the common name Williamson's. The species was named after Lieutenant Robert Stockton Williamson (1824–1882), who conducted early railroad surveying expeditions in the West Coast states (Mearns and Mearns 1992a), including the expedition upon which John Newberry collected the first male specimen.
Williamson's Sapsucker populations were stable overall from 1966-2009 but declined on Breeding Bird Survey (BBS) routes in Oregon (Sauer et al. 2011). In Canada, the species' status is Endangered because of small populations and habitat loss of mature western larch (Larix occidentalis) forests, a key habitat for the species (COSEWIC 2005).