The Boreal Owl (Aegolius funereus), known as Tengmalm's Owl in Eurasia, occupies boreal and subalpine forests in a circumboreal distribution from Scandinavia eastward across northern Siberia to Alaska, and across Canada and extreme northern portions of the U. S. to the Atlantic. Scattered populations extend southward into the subalpine forests of the Rocky Mountains, the Cascade ranges, and various mountain ranges in Eurasia.
Although well-studied in Scandinavia, where it is the most abundant forest owl, this species is less well known in North America. Where abundant, it is an important predator of forest rodents; skeletal asymmetry of its ears is pronounced and aids in precise location of such concealed prey. Female Boreal Owls are noticeably larger than males; indeed this species shows the most extreme reverse sexual dimorphism of any North American owl.
Although the Boreal Owl is not threatened on a global scale, concern exists for its populations in some areas, especially isolated montane populations south of continuous boreal forest. An obligate cavity nester, a majority of individuals in some European populations nest in artificial boxes due to loss of natural cavities after timber harvest.
Note: throughout this paper, measures of variation are 95% confidence intervals when indicated as CI and one standard error when indicated SD.