This Hawk was found breeding in the Humboldt Valley by Mr. C. S. M‘Carthy, on the Yellowstone by Mr. Hayden, at Fort Benton by Lieutenant Mullan, at Fort Resolution by Mr. Kennicott, at Fort Rae and at Fort Simpson by Mr. Ross, at La Pierre House by Lockhart, and on the Lower Anderson by Mr. MacFarlane.

Genus NISUS, Cuvier.

Gen. Char. Form slender, the tail long, the wings short and rounded, the feet slender, the head small, and bill rather weak. Bill nearly as high through the base as the length of the chord of the culmen, its upper outline greatly ascending basally; commissure with a prominent festoon. Superciliary shield very prominent. Nostril broadly ovate, obliquely horizontal. Tarsus longer than the middle toe, the frontal and posterior series of regular transverse scutellæ very distinct, and continuous, sometimes fused into a continuous plate (as in the Turdinæ!). Outer toe longer than the inner; claws strongly curved, very acute. Wing short, much rounded, very concave beneath; third to fifth quills longest; first usually shortest, never longer than the sixth; outer three to five with inner webs cut (usually sinuated). Tail long, nearly equal to wing, usually rounded, sometimes even, more rarely graduated (Astur macrourus) or emarginated (some species of subgenus Nisus).

Subgenera.

Less than one third of the upper portion of the tarsus feathered in front, the feathering widely separated behind; frontal transverse scutellæ of the tarsus and toes uninterrupted in the neighborhood of the digito-tarsal joint, but continuous from knees to claws. Tarsal scutellæ sometimes fused into a continuous plate … Nisus.

More than one third (about one half) of the upper portion of the tarsus feathered in front, the feathering scarcely separated behind; frontal transverse scutellæ of the tarsus and toes interrupted in the region of the digito-tarsal joint, where replaced by irregular small scales. Tarsal scutellæ never fused … Astur.

The species of this genus are exceedingly numerous, about fifty-seven being the number of nominal “species” recognized at the present date. Among so many species, there is, of course, a great range of variation in the details of form, so that many generic and subgeneric names have been proposed and adopted to cover the several groups of species which agree in certain peculiarities of external structure. That too many genera and subgenera have been recognized is my final conclusion, after critically examining and comparing forty of the fifty-seven species of Gray’s catalogue (Hand List of Birds, I, 1869, pp. 29–35). The variation of almost every character ranges between great extremes; but when all the species are compared, it is found that, taking each character separately, they do not all correspond, and cross and re-cross each other in the series in such a manner that it is almost impossible to arrange the species into well-defined groups. From this genus I exclude Lophospiza, Kaup (type, L. trivirgatus); Asturina, Vieill. (type, A. nitida); Rupornis, Kaup (type, R. magnirostris); Buteola, Dubus (= Buteo, type, B. brachyura, Vieill.); included by Gray under Astur, as subgenera, and Tachyspiza, Kaup (type, T. soloensis); and Scelospiza, Kaup (type, S. francesii); which are given by Gray as subgenera of Micronisus, Gray (type, Accipiter gabar), the species of the typical subgenus of which, as arranged in Gray’s Hand List, I refer to Nisus. All these excluded names I consider as representing distinct genera.

The species of this genus are noted for their very predatory disposition, exceeding the Falcons in their daring, and in the quickness of their assault upon their prey, which consists chiefly of small birds.