A fairly large hawk with a long tail, rounded wing tips, and a conspicuous pale eyebrow; adult has dark crown, blue-gray back, white underparts with dense gray barring, and conspicuous fluffy white undertail coverts; immature is brown above, buffy below, with dense blurry streaking, undertail coverts are dark-streaked, and tail has wavy dark bands bordered with white and a thin white tip; total length is 53-66 cm, with females averaging lager than males (NGS 1983).
Short General Description
A fairly large hawk; male 55 cm in length with a wingspan of 98-104 cm, female 61 cm in length with a wingspan of 105-115 cm (Squires and Reynolds 1997). Male is brown-gray to slate gray on back, head with black cap and pronounced white supercilary line. Undersides are light gray with fine horizontal vermiculations and fine black vertical streaks. Long, rounded tail, white undertail coverts, dark gray above with 3-5 dark bands and a think, white terminal band (reduced or absent with wear). Female is similar to male but browner on back and more heavily marked on underside, sometimes appearing barred. Feet, cere, toes, legs, and mouth-lining are yellow, eyes are red. Juveniles dark brown to brown-black on back with buff white and cinnamon streaks. Undersides buff white with thick cinnamon to blackish brown streaks on throat. Tail is dark brown with wavy dark-brown bands that are bordered by thin whitish bands, forming a zigzag pattern.
Migration
true - true - false - Generally a permanent resident or conducts only short-distance movements over most of range, but periodically has irruptions of movement out of northern portions of range. Fall migration appears to be influenced by prey availability (Squires and Reynolds 1997). For example, in Yukon Territory, Canada, year-round residents are abundant when snowshoe hares (LEPUS AMERICANUS) are abundant, but scarce in winter when hare population is low (Doyle and Smith 1994). Approximately once per decade, large numbers migrate southward, apparently in response to a decline in prey populations, particularly snowshoe hares and ruffed grouse (BONASA UMBELLUS; Bent 1937, Doyle and Smith 1994, Mueller et al. 1977, Squires and Reynolds 1997). Depending on location and year, fall movements begin in late August through September, peak in late September through mid-November, and typically end in December. Spring movements, which are less pronounced, begin in late February and continue through late May. Movement routes are poorly defined, particularly in the western U.S. In the eastern U.S., migrates along the Great lakes, the Appalachian Mountains and the Atlantic coast (Squires and Reynolds 1997). Some birds make extensive movements; four individuals, banded in Minnesota, were recovered up to 2400 kilometers away in British Columbia (Evans and Rosenfield 1985, cited in Squires and Reynolds 1997; Campbell et al. 1990). Other birds, however, undergo short movements from one elevation to another (Squires and Reynolds 1997).