This true arctic species, which evolved under prevailing northern conditions, exhibits a high degree of plumage variation, from extremely dark individuals to more patterned birds often termed “light morphs.” It has an extensive panboreal breeding range, with populations in taiga and tundra regions of both the Old World and the New World.
In North America, Rough-legged Hawks breed in tundra or taiga in arctic and subarctic Alaska and Canada and migrate across the boreal forest to winter in open country of southern Canada and the northern United States. A cliff-nesting species, this hawk is likely limited in distribution and numbers in many areas by the availability of suitable nesting sites. Occasionally, pairs exploit areas without cliffs by nesting in trees at the fringe of boreal forest or on human-made structures. In winter, these hawks concentrate in open areas reminiscent of their tundra summer haunts, including pastures, marshy areas, and wet meadows.
The breeding-season diet of the Rough-legged Hawk consists mostly of small rodents such as lemmings (e.g., Lemmus and Dicrostonyx) and voles (e.g., Microtus and Clethrionomys), although arctic ground squirrels (Spermophilus parryii) and a variety of birds, especially ptarmigan (Lagopus), are also eaten. Well adapted to open country, this hawk hunts from the wing, hovering in the breeze or in updrafts above hills and cliffs. The number of breeding pairs and their reproductive performance fluctuate considerably with changes in prey abundance, encouraging speculation that Rough-legged Hawks move nomadically in search of prey, but this is unproven. The winter diet is roughly similar to that of summer, although carrion can be important when snow limits the availability of small-mammal prey. Winter populations also fluctuate regionally, with “invasions” occurring in areas where small-rodent prey are abundant. As in summer, however, it is not known if individuals move in quest of prey. Despite their preference for voles and mice, Rough-legged Hawks have not escaped persecution directed at other raptors that prey on poultry or game species. Although attitudes toward raptors have greatly improved in recent decades, the affinity of this hawk for open country and its approachable nature make it vulnerable to shooting.
Studies on breeding grounds in northern Alaska and Canada ( White and Cade 1971 , Mindell et al. 1987 , Poole and Bromley 1988a , Swem 1996 ) have focused on breeding phenology, population fluctuations, and diet; studies on wintering grounds in California, Idaho, Montana, Illinois, Colorado, and Nebraska ( Mathisen and Mathisen 1968 , Schnell 1968 , Watson 1984 , Garrison 1993 , Olson and Arsenault 2000 ) have focused on population fluctuations, diet, and sexual differences in the distribution of individuals on wintering grounds. In the future, studies of reproductive and population biology, migratory behavior, and the fidelity of individuals to breeding and wintering areas should provide important new information on the biology of the species.