The species does not vary in plumage or size across North America (see Bendire 1892, Ridgway 1914, Johnsgard 1988), but birds across Eurasia are paler and white, with sparser barring ventrally and more white intermixed in the crown, although a population in central Asia is as dark dorsally (but has the ventrum and whites more like others in Eurasia). In Eurasia, pigmentation depth, the extent of white spotting, and size vary, although pronounced individual variation obscures geographic patterns (Cramp 1985). Individuals from Siberia average slightly paler and larger, as measured by wing length, yet some Scandinavian birds are equally pale and size differences are slight (Cramp 1985, Voous 1988); moreover, some birds from e. Siberia and Lake Baikal are dark.
Three subspecies, following Peters (1940); see also Am. Ornithol. Union (1957), Dement'ev and Gladkov (1966), and Cramp (1985). Marked individual variation may make diagnosis difficult (Dement'ev and Gladkov 1966), enough so that Voous (1990) considered subspecific differences to be trivial.
S. u. caparoch Müller, 1776. Includes S. u. hudsonia (Gmelin, 1788); S. u. hudsonica (Bonaparte, 1824); and Strix canadensis Stephens, 1826. Largely resident across n. North America, from central Alaska east to ne. Canada [type locality = Hudson Bay]; in winter some individuals wander south into the conterminous United States, with records south to Oregon, the n. Rocky Mts., and New York, and this subspecies has occurred farther afield in Great Britain, on the Canary Is., and possibly on the Chukchi Peninsula of e. Siberia. Dorsum dark brown; crown extensively black with some white spotting; white of scapulars broken up by black or fuscous; ventral barring dense (i.e., the dark bars are wide); averages larger.
S. u. ulula (Linnaeus, 1758). Includes Strix funerea Linnaeus, 1758; Ulula flammeata Frisch, 1766; Strix nisoria Meyer, 1810; S. u. doliata (Pallas, 1811); Strix borealis Lesson, 1830; S. u. pallasi Buturlin, 1907; and S. u. orokensis Stachanov, 1931. Largely resident across n. Eurasia from Scandinavia east to e. Siberia (Kamchatka and Sakhalin) and south to Tarbagatay [type locality = Sweden]; vagrant to w. Europe and to St. Michael I. off w. Alaska. Similar to S. u. caparoch, but dorsum paler brown, crown about equally black and white, white of scapulars more continuous, and ventral barring sparse; averages smaller.
S. u. tianschanica Smallbones, 1906. Includes S. u. korejewi Zarudnyi and Loudon, 1907. Resident in mountains of central Asia in se. Kazakhstan, e. Kyrgyzstan, and nw. China [type locality = Tien Shan, China]. Like S. u. caparoch, but ventral barring sparse (like S. u. ulula); size intermediate between S. u. caparoch and S. u. ulula (Voous 1988), although wings average ~1 cm longer than on other subspecies (Dement'ev and Gladkov 1966, Cramp 1985).
Monophyly of the family Strigidae, the typical owls (as opposed to the barn owls, family Tytonidae), has never been questioned-it is clear that, collectively, owls form a natural clade, and the typical owls form a clade separate from the barn owl radiation. Both osteology (Ford 1967) and neutral genetic markers (Sibley and Ahlquist 1990, Wink et al. 2009) placed Surnia ulula in subfamily Surniinae, with Glaucidium (the pygmy owls), Athene (the little owls, including A. cunicularia, the Burrowing Owl), Micrathene (the Elf Owl), and Aegolius (the forest owls). Karyotype also aligns these genera (Schmutz and Moker 1991). Within this subfamily, the monotypic Surnia ulula is basal to the Glaucidium radiation, and in many ways the species is much like an overgrown pygmy owl (Voous 1988).