Aegolius is a genus of small true owls. The genus name is from Latin, aegolius, is a type of screech owl that was thought to be a bird of ill omen.
The genus Aegolius was introduced in 1829 by the German naturalist Johann Jakob Kaup with the boreal owl (Aegolius funereus) as the type species.[2][3] The genus name is Latin for a screech owl, the word came from the Ancient Greek aigōlios meaning "a bird of ill omen".[4] In Greek mythology, Aegolius was originally a man whom Zeus transformed into an owl.[5]
Aegolius owls are small, stout, short-tailed, and broad-winged, with large, round facial discs. The species varies from 18 to 27 cm in length. These owls take mainly rodents and other small mammals as their prey, but also feed on birds, bats, insects, and other invertebrates according to what is available. They 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. They breed in tree holes, laying several eggs. They have repetitive whistling calls in the breeding season.
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. 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. They are mainly resident birds, but the northern species at least 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 have a very diverse mating system containing multiple strategies. Although there are five species within this genus, one of which is extinct, the majority of the information known about the mating system is based on research from the boreal owl (A. funereus) and the northern saw-whet owl (A. acadicus) in North America.[7] Both species have been shown to exhibit both monogamous and polygamous lifestyles with a focus on bigyny.[8] However, the boreal owl has been shown to exhibit biandry as well.[9]
Mate choice in this genus is largely related to auditory vocalizations (a form of sensory exploitation). A quiet species through most of the year, the northern saw-whet owl male produces a bell-like or “series song” advertisement call.[10][11] Generally, it can be heard early morning or on bright nights from March – May (the breeding season) to attract females.[12] The calls are believed to be relatively different between individuals, suggesting they may play a role in individual identification by the females.[13] After pairing with a mate, vocalizations are very limited and appear to be related to territory or offspring defence.[14]