_Milvus leucurus_, VIEILL. Nouv. Dict. Hist. Nat. XX, 556, 1816; Enc. Méth. III, 1205, 1823. _Elanoides leucurus_, VIEILL. Enc. Méth. III, 1205, 1823. _Elanus leucurus_, BONAP. Eur. & N. Am. Birds, p. 4, 1838; Consp. Av. p. 22, 1850.—GRAY, Gen. B. fol. sp. 4, 1844; List B. Brit. Mus. p. 46, 1844.—RICH. SCHOMB. Reis. Brit. Guiana, p. 735.—CASS. B. Cal. & Tex. p. 106, 1854; Birds N. Am. 1858, 37.—KAUP, Monog. Falc. Cont. Orn. 1850, p. 60.—HEERM. P. R. R. Rept. VII, 31, 1857.—COOP. & SUCK. P. R. R. Rept. XII, ii, 149, 1860.—COUES, Prod. Orn. Ariz. p. 12, 1866.—STRICKL. Orn. Syn. I, 138, 1855.—GRAY, Hand List, I, 28, 1869. _Falco melanopterus_, BONAP. Journ. Ac. Phil. V, 28; Ann. Lyc. N. Y. II, 31; Isis, 1832, p. 1137. _Milvus dispar_, LESS. Man. Orn. I, 99, 1828. _Falco dispar_, BONAP. Am. Orn. pl. xi, f. 1, 1825; Ann. Lyc. N. Y. II, 435.—AUD. Am. B. pls. cccli, ccclvii; Orn. Biog. IV, 367, 1831.—TEMM. pl. cl. 319 (_Juv._).—JAMES. (WILS.) Am. Orn. IV. 13, 1831. _Elanus dispar_, CUV. Reg. An. (ed. 2), I, 334, 1829.—LESS. Tr. Orn. p. 72, 1831.—JARD. (WILS.) Am. Orn. III, 378, 1832.—BRIDG. Proc. Zoöl. Soc. pt. ii, p. 109; Ann. Nat. Hist. XIII, 500.—AUD. Syn. B. p. 13, 1831.—BREW. (WILS.) Synop. p. 685, 1852.—NUTT. Man. p. 93, 1833. _E. leucurus_, BREWER, Oölogy. SP. CHAR. _Adult._ Upper surface, including occiput, nape, interscapulars, scapulars, rump, upper tail-coverts, and wings (except lesser and middle coverts), soft, delicate, rather light bluish-cinereous, becoming gradually white on anterior portion of the head above. Rest of the head, with the tail, lining of the wing, and entire lower parts, pure white, sometimes with a very faint tinge of pale pearl-blue, laterally beneath; two middle tail-feathers ashy, but much lighter than the rump; shafts of tail-feathers black, except toward ends. Bristly loral feathers (forming ante-orbital spot, extending narrowly above the eye), a very large patch on the shoulder, covering lesser and middle wing-coverts, and large quadrate spot on under side of wing (on first row of primary coverts), deep black. Under side of primaries deep cinereous (darker than outer surface); under surface of secondaries nearly white. Second quill longest; third scarcely shorter (sometimes equal, or even longest); first longer than fourth. Tail slightly emarginated, the longest feather (next to outer) being about .50 longer than the middle, and .60 (or more) longer than the lateral, which is shortest. _Male._ Wing, 12.50; tail, 7.10; tarsus, 1.20; middle toe, 1.15. _Female._ Wing, 12.80; tail, 7.10; tarsus, 1.45; middle toe, 1.35. Specimens not perfectly adult have the primary coverts, secondaries, and inner primaries, slightly tipped with white. Still younger individuals have these white tips broader, the tail more ashy, and the upper parts with numerous feathers dull brown, tipped narrowly with white; the breast with sparse longitudinal touches of brownish.