The northern saw-whet owl was formally described in 1788 by the German naturalist Johann Friedrich Gmelin in his revised and expanded edition of Carl Linnaeus's Systema Naturae.

[3] Gmelin based his description on the "Acadian owl" from Nova Scotia that had been described and illustrated in 1781 by the English ornithologist John Latham in his multi-volume work A General Synopsis of Birds.

[4][5] The northern saw-whet owl is now one of five species placed in the genus Aegolius that was introduced in 1829 by the German naturalist Johann Jakob Kaup.

[6][7] The genus name is Latin for a screech owl, the word came from the Ancient Greek aigōlios meaning "a bird of ill omen".

[13] Northern saw-whet owls do not exhibit sexual dimorphism in their plumage, so are often sexed by size dimorphism—females are larger than males.

[16] The northern saw-whet owl has a round, light, white face with brown and cream streaks; they also have a dark beak and yellow eyes.

[19]  A. a. brooksi is identified by a darker, buffier plumage[17] and has been proposed as a separate species, the Haida Gwaii saw-whet owl.

[21] These calls are usually produced by males searching for mates, so they can be heard more often beginning in January and continuing through the end of the breeding season in early July.

Non-vocal sounds such as bill snapping are used as a warning call by adults, juveniles and nestlings usually when approached up close or when in the hand.

[22][27] Wintering birds prefer conifer stands or shrub thickets near water, but may occasionally be found in mixed-deciduous forest.

Their range covers most of North America south of the boreal forest, including southeastern and southcentral Alaska, southern Canada, most of the United States and the central mountains in Mexico.

The second population is in the higher elevations (>4000 ft) of the southern Appalachian Mountains of western North Carolina, eastern Tennessee, and southwestern Virginia.

Because the sound reaches the ears at a different time and is of different intensity, the northern saw-whet owl can very precisely localize its prey.

Specifically in their Wisconsin study, the Swengels counted saw-whet owls as most often eating deer mice (Peromyscus; ~68% of captured prey), voles (Microtis pennsylvunicus and M. ochrogaster; ~16%), and shrews (~9%; Blarina brevicauda and Sorex cinereus).

[39] A similar study by Holt and Leroux (1996) in Montana found saw-whet owls eating more voles (60%) than other mammal species.

[40] Engel et al. (2015) also found in the saw-whet owl a strong preference for small mammals (89%), with 55% of prey being two species of voles.

An adult female northern saw-whet owl was found dehydrated and hungry within the wrapped branches of the Rockefeller Center Christmas Tree during its installation on November 16, 2020.

The feathered stowaway, named Rockefeller (Rocky), endured the three-day road trip and generated much public interest and media coverage.