Boreal owl

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Boreal owl
Parluggla, Varberg, Hallands Lan, Sverige.jpg
A. f. funereus, Hallands Län, Sweden
Boreal Owl (Aegolius funereus) (8615263478).jpg
A. f. richardsoni, Denali, Alaska
CITES Appendix II  [2]
Scientific classification OOjs UI icon edit-ltr.svg
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Strigiformes
Family: Strigidae
Genus: Aegolius
Species:
A. funereus
Binomial name
Aegolius funereus
Subspecies

7, see text

AegoliusFunereusIUCN.svg
Range of A. funereus
  Resident
  Non-breeding
Synonyms [3]
  • Nyctala tengmalmi(Gmelin, 1788)
  • Cryptoglaux tengmalmi(Gmelin, 1788)
  • Glaux funerea(Linnaeus, 1758)
  • Strix funereaLinnaeus, 1758
  • Strix tengmalmiGmelin, 1788

The boreal owl or Tengmalm's owl (Aegolius funereus) 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. [3] [4]

Contents

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, [5] and only flying off if approached to within 3–5 metres. [6]

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. [7] 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 (a synonym of A. f. funereus) as the type species. [8] [9]

Subspecies

Currently, seven subspecies are accepted, [9] though there is no genetic basis for more than two subspecies, one (nominate A. f. funereus, Tengmalm's owl) in the Old World, and one (A. f. richardsoni, boreal owl or Richardson's owl) in the New World. [10] [11] 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, [12] and genetics; [13] but this has not yet been widely followed. [9] [14]

It is the only species in its genus in Europe and Asia. Congeners in the New World include northern saw-whet owl (A. acadicus) in southern Canada and the United States (with some overlap in range), unspotted saw-whet owl (A. ridgwayi) in southern Mexico and Guatemala, and buff-fronted owl (A. harrisii) in South America. [15]

Fossil history

Remains of the boreal owl dating back to the Pleistocene era were found in a cave in southern New Mexico. [16] Bones were found in a pueblo in north-central New Mexico that has been inhabited from around A.D. 1250 to the present. [17]

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". [18]

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 21–25 cm (8–10 in) in length, with a wingspan of 55–58 cm (22–23 in), while females are larger, 25–28 cm (10–11 in), with a wingspan of 59–62 cm (23–24 in). Females, weighing 132–215 g (4.7–7.6 oz) are heavier than males, which weigh 93–139 g (3.3–4.9 oz).

Adult identification

The plumage is brown above, with conspicuous pearly-white spots on the back and wings (giving it its Danish name of perleugle, "pearl owl"), 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. [15] Unlike other owl species, these spots remain separated even when the tail is folded closely together. [6] The beak is light yellow, rather than dark like its relative the northern saw-whet owl. [19] The ears are asymmetrical in multiple bones, which enhances their hunting success. The flight is relatively noiseless and straight. [20]

Juvenile identification

Juvenile A. f. funereus, Sweden Aegolius funereus juv.JPG
Juvenile A. f. funereus, Sweden

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. [15]

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. [6] 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. [6] Males in Idaho moult from June to October, while females do so from May to October.[ citation needed ]

Vocal behaviour

A. f. funereus calling, Czech Republic

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: [21] [22]

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. [15]

Calls are similar in sound to the "winnowing" of the North American Wilson's snipe. [23] [24]

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. [25] Breeding habitats include boreal forests of black and white spruce, [25] 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. [15] This species can travel long distances but are known to breed within 500 metres of their natal sites. [15] 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. [26] [27] [28]

Behaviour

Breeding

Egg, Collection Museum Wiesbaden Aegolius funereus MWNH 0657.JPG
Egg, Collection Museum Wiesbaden

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. [29] 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. [30] The clutch is usually 3-7 eggs which are laid at 2-day intervals. They are glossy white and measure 33 mm × 26 mm (1.3 in × 1.0 in) and weigh around 11.7 g (0.41 oz). 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. [15] 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. [31] [15] Breeding densities fluctuate based on prey availability, ranging from 0.6 to 26 nests per square kilometre across different regions. [32] [33]

Food and feeding behaviour

This small owl eats mainly voles and other mammals (such as mice, chipmunks, other squirrels, gophers, moles, shrews and bats), [34] [35] but also birds as well as small amphibians, [35] insects (such as beetles) [35] 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. [34] Birds preyed upon include dark-eyed juncos, American robins and redpolls. [34] 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. [36]

Threats

Biological resource use - timber harvest

Logging activities have a negative impact on prey availability, foraging efficiency and suitable nesting sites. [37] 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. [38] The slow forest succession in spruce-fir ecosystems disrupts the recovery of critical habitat which impacts the species' ability to thrive. [25] The decline of the black woodpecker (Dryocopus martius) in Eurasia has also led to a reduced number of nesting cavities. [39]

Predation

Marten species (Martes spp.) and tawny owls (Strix aluco) are the predominant predators of boreal owl owlets and adult females at nest locations, with martens preying on 48% of Norwegian clutches. [40] Other predators include American red squirrels (Tamiasciurus hudsonicus), Cooper's hawk (Astur cooperii), American goshawk (Astur atricapillus), Eurasian goshawk (Astur gentilis) great horned owl (Bubo virginianus) and ural owl (Strix uralensis). [41] Moreover, Eurasian nuthatches (Sitta europaea) have been observed blocking the entrance of nests with mud, occasionally trapping females inside and starving them to death.

Mortality

Boreal owl mortality rates vary by region and age. In Idaho, adult annual survival rate was 46%, [42] while in Finland, first-year males had a survival rate of 50%, and adult males 67%. Most fledgling males (78%) died before their first breeding attempt. [43] Overall, adult survival ranges from 62% to 72% across studies. [44] [45] Moreover, the average lifespan of breeding males is 3.5 years, and can go up to 11 years. Ringed boreal owls have been known to live up to 16 years.

Conservation status

The current population trend is stable with 730,000 - 1,810,000 mature individuals: the species is currently listed under Least Concerned under CITES Appendix II, the EU Birds Directive Appendix I, and Raptors MOU Category 3. [1] Nevertheless, the species' lifestyle makes it hard to monitor the decline in some subpopulations. The USDA Forest Service Regions 1, 2, 3, 4 and 9 designated the species as a "sensitive species". [15] In Europe, systematic breeding surveys are being conducted in order to monitor its population status. [46] Nest boxes are implemented to create suitable nesting sites and ensure reproductive success. [25] The preservation of old-growth forests (with selective tree harvest to allow tree removal while maintaining suitable habitats [15] ) and protection of black woodpeckers (Dryocopus martius), that provide nesting cavities, are mandatory for the species' long-term survival.

References

  1. 1 2 BirdLife International (2021). "Aegolius funereus". IUCN Red List of Threatened Species . 2021 e.T22689362A201898946. doi: 10.2305/IUCN.UK.2021-3.RLTS.T22689362A201898946.en . Retrieved 12 November 2021.
  2. ↑ "Appendices | CITES". cites.org. Retrieved 2022-01-14.
  3. 1 2 "Aegolius funereus". Avibase.
  4. ↑ Beolens, Bo; Watkins, Michael (2003). Whose bird?: Men and women commemorated in the common names of birds. London: Helm. ISBN   0-7136-6647-1.
  5. ↑ Jonsson, Lars (1992). Birds of Europe. London: Helm. p. 322. ISBN   0-7136-8096-2.
  6. 1 2 3 4 Cramp, Stanley, ed. (1985). "Aegolius funereus Tengmalm's Owl". Handbook of the birds of Europe the Middle East and North Africa: the Birds of the Western Palearctic. Vol. 4. Oxford: Oxford University Press. pp. 606–616. ISBN   0-19-857507-6.
  7. ↑ Linnaeus, Carl (1758). Systema Naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis (in Latin). Vol. 1 (10th ed.). Holmiae (Stockholm): Laurentii Salvii. p. 93.
  8. ↑ Kaup, Johann Jakob (1829). Skizzirte Entwickelungs-Geschichte und natürliches System der europäischen Thierwelt (in German). Vol. c. 1. Darmstadt: Carl Wilhelm Leske. p. 34.
  9. 1 2 3 Gill, Frank; Donsker, David; Rasmussen, Pamela, eds. (January 2025). "Owls". IOC World Bird List Version 15.1. International Ornithologists' Union. Retrieved 10 January 2026.
  10. ↑ E. Koopman, Marni; B. McDonald, David; D. Hayward, Gregory; Eldegard, Katrine; A. Sonerud, Geir; G. Sermach, Sergey (2005). "Genetic similarity among Eurasian subspecies of boreal owls Aegolius funereus". Journal of Avian Biology. 36 (3): 179–183. doi:10.1111/j.0908-8857.2005.03509.x. ISSN   0908-8857 . Retrieved 2026-01-11.
  11. ↑ Homel; Nikiforov; Kheidorova; Valnisty (2020). "New Data on Phylogeography of the Boreal Owl Aegolius funereus in Eurasia". Zoodiversity. 54 (6): 523–534. doi:10.15407/zoo2020.06.523 . Retrieved 2026-01-10.
  12. ↑ Robb, Magnus; Approach, The Sound (2015). Undiscovered Owls. Poole Dorset: The Sound Approach. ISBN   978-90-810933-7-8.
  13. 1 2 3 "The OSME Region List of Bird Taxa: Part A: Non-passerines. Version 10.2: January 2025" (PDF). The Ornithological Society of the Middle East, the Caucasus and Central Asia (OSME). Retrieved 2026-01-10.
  14. ↑ AviList Core Team (2025). "AviList: The Global Avian Checklist, v2025". doi: 10.2173/avilist.v2025 . Retrieved 10 January 2026.
  15. 1 2 3 4 5 6 7 8 9 10 Hayward, G. D.; Hayward, P. H. (2020-03-04), Billerman, Shawn M. (ed.), "Boreal Owl (Aegolius funereus)", Birds of the World, Cornell Lab of Ornithology, doi:10.2173/bow.borowl.01
  16. ↑ "Google Scholar". scholar.google.com. Retrieved 2024-10-14.
  17. ↑ Emslie, Steven D. (1981-09-01). "Birds and prehistoric agriculture: The New Mexican pueblos" . Human Ecology. 9 (3): 305–329. Bibcode:1981HumEc...9..305E. doi:10.1007/BF00890740. ISSN   1572-9915.
  18. ↑ Jobling, James A. (2010). The Helm Dictionary of Scientific Bird Names. London: Christopher Helm. pp.  33, 166. ISBN   978-1-4081-2501-4.
  19. ↑ Bull, Farrand (1994). National Audubon Society Field Guide to North American Birds: Eastern Region. New York: Alfred A. Knopf. p. 554. ISBN   0-679-42852-6.
  20. ↑ Svensson, Lars; Mullarney, Killian; Zetterström, Dan (2009). Collins Bird Guide (2nd ed.). London: HarperCollins. p. 226. ISBN   978-0-00-726814-6.
  21. 1 2 3 4 5 Meehan, R. H. (1980). Behavioral significance of Boreal Owl vocalizations during breeding season. Master's Thesis, Univ. Alaska, Fairbanks.
  22. 1 2 3 Bondrup-Nielsen, S. (1984). "Vocalizations of the boreal owl, Aegolius funereus richardsoni, in North America". Canadian Field-Naturalist. 98 (2): 191–197. doi: 10.5962/p.355125 .
  23. ↑ Alaska Department of Fish and Game The Boreal Owl. , Retrieved on 10 May 2013.
  24. ↑ "Boulder County Nature Association" . Retrieved 10 May 2013.
  25. 1 2 3 4 Korpimäki, Erkki; Hakkarainen, Harri (2012-09-27). The Boreal Owl: Ecology, Behaviour and Conservation of a Forest-Dwelling Predator (1 ed.). Cambridge University Press. doi:10.1017/cbo9780511844164. ISBN   978-0-511-84416-4.
  26. ↑ Löfgren, O.; Hörnfeldt, B.; Carlsson, B. -G. (1986-06-01). "Site tenacity and nomadism in Tengmalm's owl (Aegolius funereus (L.)) in relation to cyclic food production" . Oecologia. 69 (3): 321–326. Bibcode:1986Oecol..69..321L. doi:10.1007/BF00377051. ISSN   1432-1939. PMID   28311331.
  27. ↑ Korpimäki, Erkki (1986-05-01). "Gradients in population fluctuations of Tengmalm's owl Aegolius funereus in Europe" . Oecologia. 69 (2): 195–201. Bibcode:1986Oecol..69..195K. doi:10.1007/BF00377621. ISSN   1432-1939. PMID   28311358.
  28. ↑ Sonerud, Geir A.; Solheim, Roar; Prestrud, Kristen (1988). "Dispersal of Tengmalm's Owl Aegolius funereus in Relation to Prey Availability and Nesting Success". Ornis Scandinavica (Scandinavian Journal of Ornithology). 19 (3): 175–181. doi:10.2307/3676555. ISSN   0030-5693. JSTOR   3676555.
  29. ↑ Journal of Raptor Research (2024-01-06). "Habitat Selection, Movements and Activity of Boreal and Saw-whet Owls". Journal of Raptor Research. 21 (1).
  30. ↑ Korpimaki, E. (1981). On the ecology and biology of Tengmalm's Owl (Aegolius funereus) in southern Ostrobothnia and Suomenselka, western Finland. Acta Univ. Ouluensis A 118 Biol. 13:1-84
  31. ↑ März, R. (1968). Der Rauhfusskauz. Wittenberg-Lutherstadt: Die Neue Brehm-Bücherei 394.
  32. ↑ Joneniaux, A. and G. Durand. (1987). Forest management and population ecology of Tengmalm's Owl. Rev. Ecol. Terre et la Vie, Supl. 4:84-87
  33. ↑ Korpimäki, Erkki; Norrdahl, Kai (1989). "Predation of Tengmalm's Owls: Numerical Responses, Functional Responses and Dampening Impact on Population Fluctuations of Microtines". Oikos. 54 (2): 154–164. Bibcode:1989Oikos..54..154K. doi:10.2307/3565261. ISSN   0030-1299. JSTOR   3565261.
  34. 1 2 3 "Aegolius funereus (Boreal owl)". Animal Diversity Web .
  35. 1 2 3 "Boreal Owl | the Peregrine Fund".
  36. ↑ Norberg, R. A. (1978-03-09). "Skull asymmetry, ear structure and function, and auditory localization in Tengmalm's owl, Aegolius funereus (Linné)" . Philosophical Transactions of the Royal Society of London. B, Biological Sciences. 282 (991): 325–410. Bibcode:1978RSPTB.282..325N. doi:10.1098/rstb.1978.0014. ISSN   0080-4622.
  37. ↑ Holt, Denver W.; Berkley, Regan; Deppe, Caroline; Enríquez, Paula L.; Petersen, Julie L.; Rangel Salazar, José Luis; Segars, Kelley P.; Wood, Kristin L.; Bonan, Arnau (2020-03-04), Del Hoyo, Josep; Elliott, Andrew; Sargatal, Jordi; Christie, David; De Juana, Eduardo (eds.), "Buff-fronted Owl (Aegolius harrisii)", Birds of the World, Cornell Lab of Ornithology, doi:10.2173/bow.bufowl1.01
  38. ↑ König, Claus; Weick, Friedhelm; Becking, Jan-Hendrik (2008). Owls of the world. A&C Black.
  39. ↑ Mikkola, Heimo Juhani (January 1983). Owls of Europe (1st ed.). A.D. & T. Poyser. ISBN   0-85661-034-8.
  40. ↑ Sonerud, G. A. (1985). "Nest Hole Shift in Tengmalm's Owl Aegolius funereus as Defence Against Nest Predation Involving Long-Term Memory in the Predator". The Journal of Animal Ecology. 54 (1): 179–192. Bibcode:1985JAnEc..54..179S. doi:10.2307/4629. JSTOR   4629. S2CID   86967254.
  41. ↑ Herrera, Carlos M.; Hiraldo, Fernando (1976). "Food-Niche and Trophic Relationships among European Owls". Ornis Scandinavica (Scandinavian Journal of Ornithology). 7 (1): 29–41. doi:10.2307/3676172. ISSN   0030-5693. JSTOR   3676172.
  42. ↑ Piorecky, Mark D.; Prescott, David R. C. (2004). Distribution, abundance and habitat selection of northern pygmy and barred owls along the Eastern Slopes of the Alberta Rocky Mountains /. [Edmonton]: Alberta Sustainable Resource Development, Fish & Wildlife Division, Resource Data and Species at Risk Section. doi:10.5962/bhl.title.113788.
  43. ↑ Korpimäki, Erkki (1992). "Fluctuating Food Abundance Determines the Lifetime Reproductive Success of Male Tengmalm's Owls". Journal of Animal Ecology. 61 (1): 103–111. Bibcode:1992JAnEc..61..103K. doi:10.2307/5513. ISSN   0021-8790. JSTOR   5513.
  44. ↑ Sonerud, Geir A.; Solheim, Roar; Prestrud, Kristen (1988). "Dispersal of Tengmalm's Owl Aegolius funereus in Relation to Prey Availability and Nesting Success". Ornis Scandinavica. 19 (3): 175. doi:10.2307/3676555. ISSN   0030-5693. JSTOR   3676555.
  45. ↑ Korpimäki, Erkki (1986). "Gradients in population fluctuations of Tengmalm's owl Aegolius funereus in Europe". Oecologia. 69 (2): 195–201. Bibcode:1986Oecol..69..195K. doi:10.1007/bf00377621. ISSN   0029-8549. PMID   28311358.
  46. ↑ British Trust for Ornithology (2019-03-21). A review of raptor and owl monitoring activity across Europe: its implications for capacity building towards pan-European monitoring.
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