_Falco æsalon_, RICH. & SWAINS. F. B. A. II, pl. xxv, 1831.—NUTT. Man. Orn. II, 558.—COUES, P. A. N. S. Philad. 1866, p. 42 (in text). _Falco_ (_Hypotriorchis_) _richardsoni_, RIDGWAY, P. A. N. S. Philad. Dec. 1870, 145. _Falco richardsoni_, COUES, Key, 1872, p. 214. SP. CHAR. Adult male like the female and young? The known stages of plumage more like the adult female and young of var. _lithofalco_ (_F. æsalon_, AUCT.) than like var. _columbarius_. _Adult male_ (Smithsonian, No. 5,171, mouth of the Vermilion River, near the Missouri, October 25, 1856; Lieutenant Warren, Dr. Hayden). Upper plumage dull earth-brown, each feather grayish-umber centrally, and with a conspicuous black shaft-line. Head above approaching ashy-white anteriorly, the black shaft-streaks being very conspicuous. Secondaries, primary coverts, and primaries margined terminally with dull white; the primary coverts with two transverse series of pale ochraceous spots; outer webs of primaries with spots of the same, corresponding with those on the inner webs. Upper tail-coverts tipped, and spotted beneath the surface, with white. Tail clear drab, much lighter than the primaries, but growing darker terminally, having basally a slightly ashy cast; crossed with six sharply defined, perfectly continuous bands (the last terminal) of ashy-white. Head, frontally, laterally, and beneath,—a collar around the nape (interrupting the brown above),—and the entire lower parts, white, somewhat ochraceous, this most perceptible on the tibiæ; cheeks and ear-coverts with sparse, fine hair-like streaks of black; nuchal collar, jugulum, breast, abdomen, sides, and flanks with a medial linear stripe of clear ochre-brown on each feather; these stripes broadest on the flanks; each stripe with a conspicuously black shaft-streak; tibiæ and lower tail-coverts with fine shaft-streaks of brown, like the broader stripes of the other portions. Chin and throat, only, immaculate. Lining of the wing spotted with ochraceous-white and brown, in about equal amount, the former in spots approaching the shaft. Inner webs of primaries with transverse broad bars of pale ochraceous,—eight on the longest. Wing-formula, 2, 3–4, 1. Wing, 7.70; tail, 5.00; culmen, .50; tarsus, 1.30; middle toe, 1.25; outer, .85; inner, .70; posterior, .50. _Adult female_ (58,983, Berthoud’s Pass, Rocky Mountains, Colorado Territory; Dr. F. V. Hayden, James Stevenson). Differing in coloration from the male only in the points of detail. Ground-color of the upper parts clear grayish-drab, the feathers with conspicuously black shafts; all the feathers with pairs of rather indistinct rounded ochraceous spots, these most conspicuous on the wings and scapulars. Secondaries crossed with three bands of deeper, more reddish ochraceous. Bands of the tail pure white. In other respects exactly as in the male. Wing-formula, 3, 2–4–1. Wing, 9.00; tail, 6.10; culmen, .55; tarsus, 1.40; middle toe, 1.51.