Williamson’s sapsucker is not only one of our most unique woodpeckers in its striking coloration, but it has an interesting history. Owing to the radical difference in appearance between the two sexes, they were for some time regarded as two distinct species. The female was the first to be described by John Cassin (1852, p. 349), based on a specimen collected by John G. Bell in Eldorado County, Calif. Under the name black-breasted woodpecker (_Melanerpes thyroideus_), Cassin describes and figures (1854) the adult female as the male of the species and says of the female: “Similar to the male, but with the colors more obscure, and the black of the breast of less extent and not so deep in shade,” which is a very fair description of the immature female. The male was discovered and described and figured by Dr. Newberry (1857, p. 89, pl. 34) under the name _Picus williamsonii_, based on a specimen collected by him on August 23, 1855, on the shores of Klamath Lake, Oreg. Baird, Cassin, and Lawrence (1860) give a very good description of an adult male, as the male of the species, but say “female with the chin white instead of red,” which, of course, is the immature male. Thus we have the adult of each sex regarded as the male of a species, and the young bird of each sex regarded as the female of a species. With careless, or improper, sexing of specimens, such an error might easily occur, but it is remarkable that it remained so long undiscovered. Baird, Cassin, and Lawrence (1860) describe the male as _Sphyrapicus williamsonii_ Baird, Williamson’s woodpecker, and the female as _Sphyrapicus thyroideus_ Baird, brown-headed woodpecker. J. G. Cooper (1870), in the Geological Survey of California, edited by Baird, follows the same error but calls the female the round-headed woodpecker. Even Baird, Brewer, and Ridgway, in their history of North American Birds, had not discovered the error, for they use substantially the same nomenclature. It remained for Henry W. Henshaw (1875) to discover the true relationship of the two supposed species and clear up the previous misunderstanding. He writes: “While near Fort Garland, I obtained abundant proof of the specific identity of the two birds in question; _williamsonii_ being the male of _thyroideus_. Though led to suspect this, from finding the two birds in suspicious proximity, it was some time before I could procure a pair actually mated. A nest was at length discovered, excavated in the trunk of a live aspen, and both the parent birds were secured as they flew from the hole, having just entered with food for the newly hatched young.” Mr. Ridgway (1877) comments on the discovery as follows: