Fewer than 200 papers have been published on the Northern Hawk Owl, most describing distribution and taxonomy (Clark et al. 1987b). More research is needed for the North American subspecies S. u. caparoch. Fundamental questions about its evolutionary origin and age as a species have yet to be adequately addressed.
The hawk owl is among several owl species that are thought to have colonized the Nearctic across Beringia from Eurasia (Voous 1988b). It lacks the extreme adaptations that are characteristic of other northern forest owls that feed exclusively on voles-e.g., the Great Gray Owl (Norberg 1987). The hawk owl may critically depend on birds in winter, when small mammals are less accessible under snow (Rohner et al. 1995).
Differences between subspecies (see Systematics, above) may relate to the species' opportunistic potential. The Northern Hawk Owl may be in the process of adapting to different prey species, such as a greater availability of larger prey-e.g., snowshoe hare in the Nearctic (Rohner et al. 1995). Comparative studies on the feeding ecology and life histories of subspecies are needed.
It is uncertain if competition with other raptors prevents the hawk owl from occurring in the main southern mountain chains (e.g., the Rockies), as do other circumboreal owls (Voous 1988b). The ecological relationship of this species with sympatric birds of prey, shrikes, and nest-providing woodpeckers needs study. Data on annual and seasonal diet variation are lacking (but see Rohner et al. 1995). Nesting ecology and behavior have been only superficially addressed. Accounts of breeding success, home range size, movements, dispersal, mortality, and hunting behavior remain largely anecdotal. Leading a Great Gray Owl-like life-nomadic, dispersive, and eruptive-the Northern Hawk Owl poses a challenge to researchers and naturalists to answer these questions. Winter hawk owl irruptions south of its breeding range are exciting events that periodically remind us how little we know about this familiar and striking sentinel of the Nearctic boreal forests.