Introduction

One of the least-studied birds of North America, the Northern Hawk Owl has a bold nature and seems to lack fear of humans, delighting birders, who often travel great distances to observe it. It is atypical in morphology and behavior compared to other Northern Hemisphere owls, resembling in many ways the Accipiter hawks—hence the name hawk owl. Primarily diurnal, this species usually perches atop prominent trees. When flying, it either glides low over the ground at high speed or flaps its pointed wings in deep, powerful, falcon-like strokes.

The only species in its genus, the Northern Hawk Owl breeds in the circumpolar boreal forest zone from Alaska eastward through Newfoundland, and from Scandinavia through Siberia. It nests in dead tree stubs or woodpecker holes, especially in open coniferous or mixed coniferous-deciduous forests, burned-over areas, or muskeg. Although this owl winters throughout its breeding range, it periodically “invades” southern Canada and the northern United States. The magnitude and extent of these winter irruptions are thought to correlate with high reproductive success followed by severe winter conditions and decreased prey availability. Early studies suggest that this owl eats primarily small rodents, but recent evidence indicates that grouse, ptarmigan, and hares comprise a greater proportion of their diet than previously thought.

All North American owls, except the Burrowing Owl (Speotyto cunicularia), show appreciable reversed sexual size dimorphism: The female is larger than the male. However, this dimorphism is not strongly pronounced in the Northern Hawk Owl. Despite its lack of skeletal ear asymmetry, it can locate and capture concealed prey up to 30 cm under snow cover. While much remains to be learned of the natural history of this species on its breeding range, there is no evidence that this species is at risk.

Most of our knowledge of the hawk owl is based on research in Fennoscandia (Mikkola 1983, Cramp 1985, Sonerud 1997). Low population density and the remoteness of its breeding range have challenged those wanting to study or observe it. Only 2 North American studies, based on a combined 11 nests, have focused on its breeding biology within the Nearctic boreal forest (Ritchie 1980, Rohner et al. 1995). These investigations described nest sites, breeding phenology and success, and diet. More information on habitat selection, home range, breeding dispersal, and winter diet is needed.

The relationships among prey availability, snow cover, and winter dispersal have not been documented for this species. The local appearance of tens or hundreds of hawk owls during winter south of its breeding range is an exciting event noted by researchers, birders, and laypersons alike. Such hawk owl “invasions” provide research opportunities and remind us how little we know about this familiar North American bird.