The species are:
- Boreal owl or Tengmalm's Owl, Aegolius funereus
- Northern saw-whet owl, Aegolius acadicus
- Unspotted saw-whet owl, Aegolius ridgwayi
- Buff-fronted owl, Aegolius harrisii
- Bermuda saw-whet owl, Aegolius gradyi (extinct)
- Aegolius martae (extinct)
These are essentially nocturnal woodland owls of temperate or colder climates; the two northern species breed in North America’s coniferous forests, and the unspotted saw-whet owl and buff-fronted owl (Central and South America respectively) are species of mountain, cloud or oak forests.
These are mainly resident birds, but the northern species at least will sometimes move south or to lower altitudes in autumn. The movements are difficult to monitor due to the problems of detecting these nocturnal owls outside the breeding season, when they are not calling.
Aegolius owls are small, dumpy, short tailed and broad winged, with large round facial discs. The species varies from 18–27 cm in length.
The Aegolius owls are dark brown or black above, and whitish or buff below, marked with streaks or spots in the two northern forms. The head is large, with yellow eyes and a well-defined facial disc. The flight is strong, agile and direct.
Aegolius owls breed in tree holes laying several eggs. They have repetitive whistling calls in the breeding season.