Justification of Red List Category
This iconic large owl of Arctic regions is listed as Vulnerable as it is undergoing rapid population declines in North America and probably also in northern Europe and Russia. Drivers of the decline are uncertain but likely include climate change affecting prey availability, as well as collisions with vehicles and infrastructure. There remains some uncertainty about the overall rate of decline, and if it proves to be even higher the species may be eligible for further uplisting to Endangered.
The population size was estimated to number approximately 200,000 individuals (Partners in Flight Science Committee 2013) (although there is some uncertainty over whether this refers to individuals or mature individuals), but recently, alternative methodologies have been presented which give far lower figures.
Potapov and Sale (2013) presented a ‘Loose Boid’ method of estimating population size. They concluded that instead of being evenly distributed across the tundra, snowy owls could be found in seven different loose 'boids': very thinly distributed groups which move throughout given areas in line with conditions, especially food availability. The largest of these boids was suggested to be in central northern Canada and could contain 4,000 pairs. In total, they estimated that, on average, each boid may contain 2,000 pairs and so the global population size would be c.14,000 pairs or 28,000 mature individuals, which fits with a maximal estimate of 14,000 females suggested by DNA analyses by Marthinsen et al. (2009). However, they also suggested that the population size could be as low as 7,000-8,000 pairs (Potapov and Sale 2013).
The European population is estimated at 700-2,300 pairs, which equates to 1,400-4,600 mature individuals (BirdLife International 2015).
High rates of population decline have been reported in at least the American and Canadian part of its range, with Rosenberg et al. (2016) estimating a 64% decline in U.S.A. and Canada between 1970 and 2014; with an estimated population in these two countries of <30,000 individuals. Extrapolating backwards this would equate to a decline of c.43.6% over 3 generations (c.24 years) in the population in these countries.
This trend data can then be used in conjunction with Potapov and Sale’s (2013) ‘Loose Boids’. By sub-dividing the global population into these thinly distributed groups and assuming 4,000 pairs are found in the central northern Canadian Boid, this would then mean that on average c.1,667 pairs are found in each other grouping. One of the other 6 boids proposed by Potapov and Sale (2013) is restricted to North America [on Wrangel Island] and one more may move into North America [ranging from the Indrigirka River in Russia to Victoria Island in Canada]. Therefore, there may be between 5,667 and 7,333 pairs in North America at a given time; equating to 6,667-8,333 pairs outside of this range. Taking a very crude view that declines have only occurred in North America, and extrapolating backwards, the population estimates for this region would equate to global population declines over the past 3 generations in the range of 25.3-30.1%. This is not the case though, as the Snowy Owl is known to have declined in the Western Palearctic (e.g. Portenko 1972; Solheim 1994, 2004; I. J. Øien in litt. 2014; T. Lehtiniemi in litt. 2017), and climate change will be likely having global impacts on this species rather than local impacts. Therefore the global decline over three generations may in fact be more similar to that for North America alone – 43.6%. It should be noted that Snowy Owl populations do fluctuate (BirdLife International 2015) and so this may affect population trend estimates. Partners in Flight (2019) currently estimate a rate of annual decline of ~1.68%, which equates to a decline of ~33.4% over three generations. As such, given the estimates presented, and the potential for threats to continue into the future then this species is likely to be undergoing global declines of 30-49% in three generations, or possibly even higher.
This species has a huge range across predominantly Arctic regions from western Scandinavia through northern Russia to Alaska (USA), northern Canada and Greenland (Denmark), and has also bred occasionally in Iceland and the UK. In winter birds move further south into the USA, northern Europe and north Asia.
The species breeds on open tundra from near tree-line to edge of polar seas, with hummocks or rocks and sparse low vegetation and dwarf shrubs and lichen; locally also coastal fields and open moorland, usually at elevations below 300 m, except in Norway (where lemmings occur only on mountains at 1,000 m or higher). In winter, also marshes, fields and dunes (Holt et al. 2017).
Lemmings (Lemmus, Dicrostonyx) and other voles (Microtus, Clethrionomys) are major food items, at times probably exploited exclusively; other prey is taken according to availability, include other mammals to size of hares, birds to size of ptarmigans (Lagopus), ducks and medium-sized geese, occasionally fish, amphibians, crustaceans and beetles (Holt et al. 2017).
It is generally monogamous and often pairs for life; but with occasional polygyny and polyandry. The nest is a shallow scrape on ground, usually in a slightly elevated site (Holt et al. 2017).
It is most active at dusk and dawn, but forages throughout the day in summer. It is mostly migratory and nomadic, with some remaining in the breeding area all year if conditions allow (Doyle et al. 2017). Movements are unpredictable and related to abundance of prey species; thought to vary considerably from region to region across polar tundra, and intensity of movements fluctuates annually (Doyle et al. 2017; Holt et al. 2017). During years of abundant prey, increased numbers of young Snowy Owls fledge and hence greater numbers therefore migrate south into the USA from their Canadian/Alaskan Arctic breeding grounds to find sufficient wintering territory, a phenomenon known as a 'snowy owl irruption' (Therrien et al. 2017; Curk et al. 2018; Santonja et al. 2019).
Harvesting of the species for food, feathers and claws by native peoples may have local impact on population, but unlikely to have any wider effect on total numbers. Many apparently die from starvation during irruptive southerly movements from arctic regions. Electrocution, aeroplane strikes, collision with vehicles and entanglement in fishing equipment have all been identified as sources of mortality in Alberta, Canada (Holt et al. 2017). Climate change has a significant impact on the onset of spring and snowmelt in the breeding areas which may change the availability of prey for the species (International Snowy Owl Working Group 2010).
Conservation and research actions underway
An International Snowy Owl Working Group gathers Snowy Owl researchers from across the range of the species (breeding and non-breeding) from 9 countries. The group is working on a thorough status assessment for the species, including population size, precise distribution, dispersal and survival rates (J. F. Therrien in litt. 2017). Satellite tracking projects in the Canadian Arctic and Norway have documented extensive individual movements, suggesting the population may have been overestimated in the past (R. Solheim and G. Gauthier in litt. 2017)
Conservation and research actions needed
Clarify population size and trend by carrying out coordinated surveys across breeding grounds during lemming peak years and repeating these in subsequent peak years. Work to enact and enforce policies that protect large birds from electrocution, vehicle and aeroplane strikes.
Text account compilers
Martin, R., Everest, J.
Ashpole, J, Butchart, S., Ekstrom, J., Gauthier, G., Holt, D., Lehtiniemi, T., O'Brien, T., Solheim, R., Symes, A., Therrien, J. & Westrip, J.R.S.
BirdLife International (2021) Species factsheet: Bubo scandiacus. Downloaded from http://www.birdlife.org on 18/10/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 18/10/2021.