Introduction

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 most 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 in 1852 and classified a female specimen as a male Black-breasted Woodpecker (Melanerpes thyroideus, or Picus thyroideus; Bent 1939, Am. Ornithol. Union 1983). This “species” underwent several subsequent name changes, including Brown-headed and Round-headed Woodpecker, or Sphyrapicus thyroideus . John Newberry discovered the male in 1857 and named it Williamson’s Woodpecker (Picus williamsonii), which was renamed Sphyrapicus williamsonii . Finally, Henry Henshaw verified that the two sexes were a single species when in 1873 he was the first to observe a mated pair, at a nest in Colorado (Bent 1939). 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 1992).

Williamson’s Sapsucker populations declined throughout their range from 1982 to 1991, with par-ticularly strong declines in the Pacific Northwest. Data are few, however, because of the sparse number of Breeding Bird Survey (BBS) routes in the western U.S.; more complete surveys are needed.