LC
Boreal Owl Aegolius funereus



Justification

Justification of Red List category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (extent of occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population trend appears to be fluctuating but stable in the long term, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.

Population justification
Rich et al. (2004) estimated the global population to number > c.1,700,000 individuals which equates to 1,130,000 mature individuals. Partners in Flight (2020) estimate the population in the USA and Canada to be 500,000 individuals and the global population to be 1,540,000 individuals, which equates to approximately 1,030,000 mature individuals. The European population is estimated at 94,600-236,000 pairs, which equates to 189,000-471,000 mature individuals (BirdLife International in prep.). Europe forms approximately 26% of the global range, so a very preliminary estimate of the global population size is 730,000-1,810,000 mature individuals, although further validation of this estimate is needed. The population is placed in the band 730,000-1,810,000 mature individuals.

Trend justification
Long-term trends are difficult to assess due to nomadism and yearly population fluctuations corresponding with rodent abundance and snow cover (Hayward & Hayward 2020; Korpimäki 2020). The population in some European countries has declined significantly since the early 1990s as a result of loss of primary habitat through clear-felling of boreal forest (Korpimäki 2020). However, recent data suggest that the European population is fluctuating, but is stable in the long-term (BirdLife International in prep.), with local range expansions/ contractions and population increases/ declines tending to balance each other out (BirdLife International in prep.; Keller et al. 2020). The population in the North Caucasus is considered to be stable (Belik & Akkiev 2019). There is very little data on North American populations, but trends are likely to be similar to those in Europe.

Ecology

Breeds in Eurasian and North American coniferous forests, particularly dense spruce forest (Korpimäki & Hakkarainen 2012). In Finland, high proportions of spruce forest and agricultural land in the landscape are associated with greater reproductive success, possibly due to higher small mammal numbers, although birds avoid hunting in large open areas such as agricultural fields (Korpimäki & Hakkarainen 2012). Old-growth forest is particularly important for lifetime reproductive success, potentially because it maintains higher prey numbers and refuges against large avian predators (Korpimäki & Hakkarainen 2012).

Threats

The breeding population of this species fluctuates with vole cycles and the depth of annual snow cover, which inhibits foraging. Forestry can reduce primary prey populations, remove forest structure necessary for foraging, and eliminate nest cavities (Holt et al. 1999). Old-growth forest is particularly important - significant declines in northern Europe since the 1990s have been associated with loss of old-growth boreal forest due to clear-felling  (Korpimäki & Hakkarainen 2012). At one time the species frequently used old Black Woodpecker (Dryocopus martius) holes in Eurasia but declines in the species has resulted in less nesting opportunities (Mikkola 1983). Tawny owls (Strix aluco) and martens (Martes spp.) are serious predators of this species and in some years the latter can destroy a high percentage of broods and kill many females on the nest. In Germany, the Nuthatch (Sitta europea) may reduce the size of nest-hole entrances with plastered mud and has even been known to wall in brooding females, resulting in the birds starving to death. It is also vulnerable to pesticides (König et al. 2008).

Conservation actions

Conservation actions underway
The species is listed on CITES Appendix II, EU Birds Directive Appendix I and Raptors MOU Category 3. Systematic breeding surveys are carried out for this species in at least 8 European countries (Derlink et al. 2018). Nestboxes are used extensively in Europe to mitigate loss of nest sites (Korpimäki & Hakkarainen 2012).

Conservation actions needed
Preserve old-growth forest, particularly spruce forest with suitable nest holes made by Black Woodpeckers.

Acknowledgements

Text account compilers
Haskell, L.

Contributors
Ekstrom, J., Ashpole, J & Butchart, S.


Recommended citation
BirdLife International (2024) Species factsheet: Aegolius funereus. Downloaded from https://datazone.birdlife.org/species/factsheet/boreal-owl-aegolius-funereus on 23/02/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from https://datazone.birdlife.org on 23/02/2024.