The northern hawk-owl was formally described by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his Systema Naturae under the binomial name Strix ulula.

[9] Three subspecies are recognised:[5] Male northern hawk-owls are generally 36–42.5 cm (14.2–16.7 in) long and weigh about 300 g (11 oz).

The underbelly is generally white or off-white which continues to the toes with brown bands on the breast and stomach.

It has been suggested that this may be because the hawk-owl may partially fill an important diurnal niche similar to that of day hunters such as hawks.

When attracting a mate the male usually lets out a rolled whistle of ulululululululul and a sound similar to tu-wita-wit, tiwita-tu-wita, wita, when perching at a potential nest site.

[13][14] The North American subspecies S. u. caparoch spans from eastern Alaska through Canada to Newfoundland and in some areas extends south into northern United States.

These southern forays into the northern United States are rare and generally occur during winter, or following an explosion in a population of prey.

[13] They are found in muskegs, clearings, swamp valleys, meadows,[13] or recently burnt areas,[15] and generally avoid dense spruce-fir forests.

[11] After calling and pairing is complete the northern hawk-owl will build a nest and start to lay eggs.

[16] For the most part, the female northern hawk-owl does the incubating of the eggs, whilst the male forages for food.

At about two weeks into the chicks' lives, the female starts to leave the nest for long spans of time (5 hours or more).

In North America the subspecies S. u. caparoch also feeds upon microtine voles, but its population is primarily based on the ten-year cycling of the snowshoe hare (Lepus americanus).

[16] A long list of others includes mice, rats, voles, lemmings, cottontails, shrews, moles, the short-tail weasel (Mustela erminea), partridge, spruce grouse (Falcipennis canadensis), doves, pileated woodpeckers (Dryocopus pileatus), sparrows, jays, robins, starlings, buntings, grackles, and finches.

[16] Due to its low density occurrence, sporadic fluctuations, and remote breeding locations,[20] the northern hawk-owl is one of the least studied and poorly understood birds in North America.

[17] Despite these low densities, the North American population is thought to be fairly large given that they occur throughout the boreal forest.

[13] However, it is unknown what effects modern forestry would have on population levels because although it would decrease nesting localities, it would simultaneously create ideal habitat for Microtus prey.

[11] Fire suppression by humans is believed to negatively affect northern hawk-owl populations by reducing open areas for hunting and dead wood to nest in.

[14] A report by the Committee On the Status of Endangered Wildlife In Canada (COSEWIC) recommended that no designation be assigned for the northern hawk-owl.

[25] Downes et al. (2000) considered the hawk-owl to be of medium concern, but with a high priority to improve monitoring.

[26] The northern hawk-owl is considered a falconry bird in Ontario, and may be used to hunt small game with a proper license.