Specimens from different regions vary but little in size. The largest are 4,198, ♀, San Francisco, Cal., winter, 16,957, ♀, Hudson’s Bay Territory, and 55,016, ♀, Mazatlan, Mexico, in which the wing ranges from 8.40 to 8.50, the tail 7.00. The smallest females are 45,826, Sitka, Alaska, and 11,791, Simiahmoo, W. T., in which the wing measures about 7.80. A female (32,499) from Orizaba, Mexico, one (8,513) from Fort Yuma, Cal., and one (17,210) from San Nicholas, Lower California, have the wing 8.00, which is about the average. The largest males are 54,336, Nulato, Alaska, 58,137, Kodiak, Alaska, 27,067, Yukon, mouth of Porcupine, and 55,017, Mazatlan, Mexico, in which the wing measures 7.00, the tail 5.60. The smallest males are 5,990, Orange, N. J., 8,514, Shoalwater Bay, W. T., 21,338, Siskiyou Co., Cal., 37,428, Orizaba, Mexico, and 5,584, Bridger’s Pass, Utah; in this series the wing measures 6.50–6.70, the tail 5.40–5.60. A specimen from Costa Rica measures: wing 6.70, tail 5.35. Thus the variation in size will be seen to be an individual difference, rather than characteristic of any region. Some immature specimens from the northwest coast of North America (as 45,828, ♂, Sitka, Rus. Am., 5,845, ♂, Fort Steilacoom, W. T., 11,791, Simiahmoo, Puget Sound, and 8,514, Shoalwater Bay, W. T.) are much darker than others, the brown above inclining to blackish-sepia; no other differences, however, are observable. An adult from the Yukon (54,337, ♀) has the rufous bars beneath remarkably faint, although well defined; another (19,384, ♀, Fort Resolution), in immature plumage, has the longitudinal markings beneath so faint that they are scarcely discernible, and the plumage generally has a very worn and faded appearance. A male in fine plumage (10,759, Fort Bridger, Utah) has the delicate reddish-rufous beneath so extended as to prevail, and with scarcely any variegation on the sides and tibiæ; the bars on the tail, also, are quite obsolete. HABITS. This species is one of the most common Hawks of North America, and its geographical range covers the entire continent, from Hudson’s Bay to Mexico. Sir John Richardson mentions its having been met with as far to the north as latitude 51°. Drs. Gambel and Heermann, and others, speak of it as abundant throughout California. Audubon found it very plentiful as far north as the southern shore of the Gulf of St. Lawrence. It has been obtained in New Mexico by Mr. McCall, in Mexico by Mr. Pease, in Washington Territory by Dr. Cooper and Dr. Suckley, in Alaska by Mr. Dall, at Fort Resolution by Mr. Kennicott, at Fort Simpson by Mr. B. R. Ross, etc. Messrs. Sclater and Salvin give it as a rare visitant of Guatemala. It has been ascertained to breed in Massachusetts, New Jersey, Wisconsin, California, and Pennsylvania, and it probably does so not only in the intervening States and Territories, but also in all, not excepting the most southern, Florida, where its nest was found by Mr. Wurdemann.