[5] The genus contains five species:[6] Aegolius owls are small, stout, short-tailed, and broad-winged, with large, round facial discs.

Three of the extant species are restricted to the New World, but the boreal owl has a circumpolar range through North America, Eurasia, the Alps, and the Rockies.

A quiet species through most of the year, the northern saw-whet owl male produces a bell-like or “series song” advertisement call.

[14] In contrast, the boreal owl produces a “staccato song” starting around dusk, and ending around midnight in the late winter and early spring.

[15][16] Some research suggests that unlike the Northern saw-whet owl, a variation of the male call persists after mating throughout the reproductive cycle for defense, aggression, or to convey information about prey.

For example, in boreal owls, the number and quality of nest holes or boxes defended by males is a factor in female reproductive effort.

Some researchers suggest they follow a similar trend to boreal owls, where the quality of nests is a direct benefit to the female and therefore a determining factor.

[19] Northern saw-whet owls' nest locations appear less linked to the availability of small mammals, as they have a greater variety of prey options and tend to be more nomadic after each breeding season.

[21] Another theory suggests that females purposely choose short-winged, light males, as they would be able to hunt at lower energetic costs and compete for territory more efficiently than larger, long-winged individuals.

[24]Parental care in the boreal and northern saw-whet owls is quite similar; females incubate and brood eggs, while males often provide protection, and resources such as food and nesting sites.

[26] This is similar to the differential allocation hypothesis, which suggests that females increase their reproductive effort overall when offspring are the result of ideal mates.

[28] Information on parental care effort is limited on the northern saw-whet owl, and likely related to male provisioning rates, the extent, however, is unclear.