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, 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 very 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.
The global population is estimated to number approximately 190,000 individuals which equates to 127,000 mature individuals (Partners in Flight Science Committee 2013). The North American population is estimated at 90,000 individuals or 60,000 mature individuals (Partners in Flight Science Committee 2013). The European population is estimated at 1,900-7,500 pairs, which equates to 3,900-15,000 mature individuals (BirdLife International 2015). Europe forms approximately 13% of the global range, so a very preliminary estimate of the global population size is 30,000-115,000 mature individuals, although further validation of this estimate is needed. The population is precautionarily placed in the band 50,000-99,999 mature individuals but the actual population could be considerably larger.
The overall trend is likely to be increasing. This species has undergone a large and statistically significant increase over the last 40 years in North America (4900% increase over 40 years, equating to a 166% increase per decade; data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven 2007). Note, however, that these surveys cover less than 50% of the species's range in North America. In Europe the population size is also estimated to be increasing (BirdLife International 2015).
The species is affected by global warming, which if it persists will continue to move the species’s range northwards (Hagemeijer and Blair 1997). Populations fluctuate in line with vole numbers (Cornulier et al. 2013). Locally, hunting may still be a threat (König 2008). It is also vulnerable to road traffic collisions and loss of habitat from forestry (Holt et al. 1999). Collisions with power lines and cables are also a threat. In North America timber harvesting, collisions with vehicles, strychnine poisoning of pocket gophers, disturbance at foraging habitats due to development of campsites, grazing, peat extraction and agriculture have all been identified as potential threats (Holt et al. 1999). The species is known to be vulnerable to West Nile Virus (Lopes et al. 2007).
Text account compilers
Butchart, S., Ekstrom, J. & Ashpole, J
BirdLife International (2021) Species factsheet: Strix nebulosa. Downloaded from http://www.birdlife.org on 24/06/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 24/06/2021.