Boreal owl


The boreal owl or Tengmalm's owl is a small owl in the "true owl" family Strigidae. It is known as the boreal owl or, more rarely, Richardson's owl after Sir John Richardson, in North America, and as Tengmalm's owl in Europe and Asia, after the Swedish naturalist Peter Gustaf Tengmalm.
Due to its nocturnal habits and preferred inaccessible taiga forest habitat, it is rarely seen by humans; when met with, it is however often extremely confiding, showing little or no fear of people, and only flying off if approached to within 3–5 metres.

Taxonomy

The boreal owl was formally described by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his Systema Naturae. Linnaeus placed it with all the other owls in the genus Strix and coined the binomial name Strix funerea. The boreal owl is now placed in the genus Aegolius that was introduced in 1829 by the German naturalist Johann Jakob Kaup with Strix tengmalmi J. F. Gmelin, 1788 as the type species.

Subspecies

Currently, seven subspecies are accepted, though there is no genetic basis for more than two subspecies, one in the Old World, and one in the New World. Some authors, notably the Ornithological Society of the Middle East, have recommended splitting the Old and New World populations as separate species due to significant differences in vocal behaviour, and genetics; but this has not yet been widely followed.
  • New World
  • *A. f. richardsoni – Alaska, northern Canada and north United States
  • Old World
  • *A. f funereus – nominate subspecies, from Scandinavia down south to the Pyrenees and east to the Urals, but not the Caucasus Mountains
  • *A. f. magnus – northeast Siberia
  • *A. f. sibiricus – southeast and south Siberia
  • *A. f. pallens – west and central Siberia, south to the central Asian mountains in Kyrgyzstan
  • *A. f. caucasicus – Caucasus Mountains, and locally in northern Turkey and the Alborz
  • *A. f. beickianus Stresemann, 1928 – northwest India to southwest China

    Related species

It is the only species in its genus in Europe and Asia. Congeners in the New World include northern saw-whet owl in southern Canada and the United States, unspotted saw-whet owl in southern Mexico and Guatemala, and buff-fronted owl in South America.

Fossil history

Remains of the boreal owl dating back to the Pleistocene era were found in a cave in southern New Mexico. Bones were found in a pueblo in north-central New Mexico that has been inhabited from around A.D. 1250 to the present.

Etymology

The genus name is Latin for a screech owl, the word came from the Ancient Greek aigōlios meaning "a bird of ill omen". The specific epithet funereus is Latin meaning "funereal".

Description

The boreal owl is small with a large head and long wings. Both sexes are similar in appearance, though females are larger. Males typically measure in length, with a wingspan of, while females are larger,, with a wingspan of. Females, weighing are heavier than males, which weigh.

Adult identification

The plumage is brown above, with conspicuous pearly-white spots on the back and wings, and whitish underparts with rust-coloured streaks. Its large head has yellow eyes and an olive-brown crown with small white droplets and larger central spots. The dull white facial disc is framed with a dark border and white "eyebrows" often described as giving the owl a "surprised" expression. The underparts are cream-white with russet-brown streaks, while the tail has olive-brown feathers with narrow bars. Flight feathers are dark olive-brown with contrasting white spots. The primaries typically have up to five rows of white spots, while the tail has three distinct rows of spots. Unlike other owl species, these spots remain separated even when the tail is folded closely together. The beak is light yellow, rather than dark like its relative the northern saw-whet owl. The ears are asymmetrical in multiple bones, which enhances their hunting success. The flight is relatively noiseless and straight.

Juvenile identification

Juveniles, at about 3 weeks old, have a completely dull, chocolate-brown upperparts and underparts, with a dark brown facial disc bordered by black and white markings near the bill. Upon fledging, their plumage remains similar but with white markings on the neck, scapulars, flight feathers, and tail. They develop a broad white "X" between the eyes and distinct white streaks along the face, with cream-coloured belly and flanks streaked in dark grey-brown.

Moults

Boreal Owls have two primary moulting phases. The first, post-juvenile moult occurs around the age of two months, between August and September. It only affects the head, body, and wing coverts. The flight feathers, tertials, greater primary coverts, and tail are unaffected. Subsequent moults are also partial, affecting all of the head, body, and wing coverts, as well as the tail, but only a portion of the flight feathers, which are replaced in a specific order each year; the sequence of the primary moult not yet fully understood. Males in Idaho moult from June to October, while females do so from May to October.

Vocal behaviour

Calls and related functions can be challenging to interpret because of their nocturnal and hard to locate lifestyle. Starting at one week old, young owls produce a harsh "peep" or "chirp" that softens after fledging. Comprehensive studies in North America have found 8 different types of calls that play an important role in communication, mating, and territorial behaviour:
  • The Primary Song - loud trill sung by males to attract females and establish territory
  • The Prolonged Song - softer courtship melody sung by males from beginning of courtship to incubation to strengthen bonds
  • The Delivery Call - soft call by male to signal the transfer of prey to a female or young
  • The Screech - a load contact call made while flying or from a perch
  • The Peeping Call - soft calls by females throughout breeding to respond to males' calls
  • The Weak Call - single note repeated rapidly from females in nest after hatching
  • The Chuuk Call - harsh response call by females on male's territory
  • The Hiss Call - defensive call by females when the nest is disturbed
Bill clapping is used as a protective sound by both adults and juveniles, and is frequently associated with vocal hiss. Young develop bill clapping at least one week before fledging. Adults reply with a bill clap when humans handle them or enter the nest cavity.
Calls are similar in sound to the "winnowing" of the North American Wilson's snipe.

Distribution and habitat

The boreal owl breeds in dense boreal, subalpine forest as well as mixed woodland. The preferred habitats are old-growth forests as they provide shelter from avian predators and an abundance of prey. Breeding habitats include boreal forests of black and white spruce, birch and aspen across northern North America and the Palearctic, and in mountain ranges such as the Alps and the Rockies. Nests can be found in aspen and mixed forests, while individuals roost in conifers of about 6 metres high. This species can travel long distances but are known to breed within 500 metres of their natal sites. Males have a higher site fidelity than females and occupy stable year-long home ranges. However, northern populations may migrate south in response to snow depth and poor prey availability, but eventually return to their breeding grounds.

Behaviour

Breeding

Boreal owls are monogamous and raise a single brood once they reach one year of age. The breeding begins with a lengthy courtship process where males sing for up to 102 days, starting from mid-February to mid-April, depending on environmental factors like prey availability and weather. The nest is usually a hole in a tree often made by a woodpecker, but the birds also readily use nest-boxes. Nesting period last for 28–36 days. The clutch is usually 3-7 eggs which are laid at 2-day intervals. They are glossy white and measure and weigh around. The eggs are incubated only by the female beginning with the second egg and incubation lasts on average 29 days for the first egg and 2.6 days less for the last one. They hatch asynchronously after 25–32 days. Females take care of the young and brood the nestlings for an average of 21 days. Food is brought by males during the first 3 weeks to the nest and female will tear apart the food for 2 weeks until young are able to eat unassisted. The young fledge after 28–36 days and become independent at 3 to 6 weeks outside the nest cavity. Breeding densities fluctuate based on prey availability, ranging from 0.6 to 26 nests per square kilometre across different regions.

Food and feeding behaviour

This small owl eats mainly voles and other mammals, but also birds as well as small amphibians, insects and other invertebrates. In North America, they have been observed preying on southern red-backed voles, western heather voles, jumping mice, northern pocket gophers and northern flying squirrels. Birds preyed upon include dark-eyed juncos, American robins and redpolls. It is largely nocturnal, though in the northernmost parts of its range, it has to hunt during daylight because of the very short nights in summer. This species is a sit-and-wait predator meaning it does not pursue animals until within 10 metres of its hunting perch. Their extreme asymmetrical skulls leads to pinpoint sound in both horizontal and vertical directions, and allows them to capture prey under snow and dense vegetation.

Threats

Biological resource use - timber harvest

Logging activities have a negative impact on prey availability, foraging efficiency and suitable nesting sites. Since the 1990s, significant population declines in northern Europe have been attributed to clear-cutting. Moreover, pesticides from agriculture and forestry effluents are damaging to the species' health. The slow forest succession in spruce-fir ecosystems disrupts the recovery of critical habitat which impacts the species' ability to thrive. The decline of the black woodpecker in Eurasia has also led to a reduced number of nesting cavities.