A suspicion that the two might eventually prove to be different plumages of one species several times arose in our mind during the course of our field-work, the chief occasion for which was the very suggestive circumstance that both were invariably found in the same woods, and had identical manners and notes, while they also agreed strictly in all the details of form and proportions, as well as in the bright gamboge-yellow color of the belly. Our theory that _thyroideus_ was perhaps the _young_, and _williamsoni_ the _adult_, proved erroneous, however; and it never occurred to us that the differences might be sexual, an oversight caused chiefly by the circumstance of our having seen in collections many specimens of _thyroideus_ with a red streak on the throat and marked as males, while the type specimen of _williamsoni_ had a white streak on the throat and was said to be a female. We were thus entirely misled by the erroneous identification of the sex in these specimens. We gave the matter up, however, only after shooting a very young specimen of what was undoubtedly _williamsoni_, and another of _thyroideus_, both of which very closely resembled the adults of the same forms, a circumstance which at once convinced us that the differences could not depend on age; so we finally concluded that the two must be distinct. All observers seem to agree that this woodpecker is confined to the higher elevations in the mountains among the pines, in sharp contrast to the haunts of the red-breasted sapsucker at lower levels among the deciduous trees. Joseph Grinnell (1908), referring to the San Bernardino Mountains, in southern California, says: “This Williamson sapsucker appeared to be restricted to the Canadian zone and upper edge of Transition. We found it only among the tamarack pines on the slopes and ridges of San Gorgonio peak, and among the silver firs, tamarack and yellow pines around Bluff lake. In the former locality the species was common for a woodpecker, especially around Dry Lake, 9,000 feet altitude, where several nests were found.” _Courtship._--Charles W. Michael (1935) noted the mating behavior of a male Williamson’s sapsucker, which had just left a fresh nest-hole, as follows: