A new study, funded in significant part by Project SNOWstorm, is the first global conservation status assessment for snowy owls. (©Jean Hall)

For the first time, an assessment of the global conservation status of snowy owls has been completed, confirming that worldwide population was badly overestimated for decades, and indicating that snowy owl populations have declined by roughly a third over the past 25 to 30 years.

The major study, which involved dozens of snowy owl researchers from five countries, all part of the International Snowy Owl Working Group (ISOWG), was led by Dr. Rebecca McCabe of Hawk Mountain Sanctuary in Pennsylvania, a core team member of Project SNOWstorm, which provided significant underwriting to make the multi-year analysis possible.

The study was published in late December in the prestigious journal Bird Conservation International, and was released as Open Access, thanks to Project SNOWstorm and its ISOWG colleagues in Norway, which jointly paid a fee to remove the journal’s paywall on this important paper, allowing anyone to read and download it for free.

The status assessment combined long-term breeding data from sites in the circumpolar Arctic, telemetry tracking data, genetics studies and survival estimates to confirm previous, much lower estimates of the global snowy owl population, to estimate declines over the past decades, and examine the threats facing this at-risk species.

The study suggests worldwide snowy owl populations have fallen by about 30 percent in the past 25 to 30 years. (©Jean Hall)

“I am thrilled to see this article published in Bird Conservation International,” Dr. McCabe said. “It was truly a collaborative effort, with 39 snowy owl researchers from around the world contributing to the first global status assessment for the species. It’s been in the works since its conception at the ISOWG meeting in 2017 when the idea was proposed. With many iterations and incorporating additional data from collaborators, we finally have a comprehensive assessment almost eight years later.”

“This paper emphasizes the vital role of collaboration and data sharing in science, and our hope is that the results from this study will inform and create effective conservation strategies for the snowy owl,” Dr. McCabe said.

As recently as 2004, scientists thought the global snowy owl population might be as high as 290,000 snowy owls, but that was before satellite tracking revealed the extent to which snowy owls are highly nomadic between breeding seasons, often moving thousands of kilometers from one summer to the next in search of abundant prey. Consequently, those previous estimates likely double- or triple-counted owls as they moved across enormous distances from year to year.

More recent estimates placed the likely total population at 14,000-28,000 breeding-age adults. This does not mean that snowy owl numbers have dropped by an order of magnitude from nearly 300,000 to fewer than 30,000, only that the much higher, earlier estimates were incorrect. Combining a variety of approaches, including recent genetic analyses that allowed scientists to estimate the owls’ current effective population size by looking at mitochondrial DNA across its range, the new study’s authors feel confident the 14,000-28,000 adult population is correct, and the 2017 reclassification of snowy owls from “least concern” to “vulnerable” by the International Union for the Conservation of Nature (IUCN) was justified.

So, there are far fewer snowy owls in the world than conservationists once thought. The same genetic analysis that supported the lower population estimate also showed that snowy owls have been in steady decline since the end of the last glacial period 20,000 years ago, with accelerated declines during past warming periods. Given the pace of climate change in the Arctic today, what is the more recent population trend? That remained unknown.

Population trends were based on data from long-term monitoring sites in the Arctic. (©Bird Conservation International)

To find out, McCabe and her team used long-term data from five sites in the Arctic, in Canada, Alaska, Fennoscandia, Greenland and Russia, dating back to the late 1980s. (Two other sites, one in Canada and one in Greenland, were excluded because they had not been monitored for more than 10 years.) The cyclical and unpredictable nature of snowy owl nesting, which depends on peaks in small mammal (primarily lemming) populations, made the analysis challenging. Into these trends were factored a variety of other variables, including estimates of annual adult survival, which is relatively high, a fact confirmed by tracking data including that from Project SNOWstorm.

The study’s authors plotted the median population trend estimate (heavy blue line) based on the five long-running monitoring stations, showing a general decline below the dashed line that represents a stable population. (©Bird Conservation International)

They concluded, however, that the global snowy owl population has continued to fall, declining by about 30 percent in the past three owl generations, at between 8 and 10.7 years per generation. If these trends continue, there may be justification for uplisting snowy owls to a higher IUCN category like  endangered, although for the time being the study’s authors recommended maintaining the “vulnerable” classification.

Most importantly, the study urged additional research into several broad areas. Scientists need a better understanding of seasonal survival rates in adults, and the rate at which nestlings survive to the fledging and post-fledging period. (This latter question is one that SNOWstorm and its ISOWG partners have started to address, tagging nestling snowy owls last summer in the Canadian Arctic.) We need a clearer picture of the age at which snowy owls typically first breed, which is largely unknown. The study recommends continued long-term monitoring on the Arctic breeding grounds, and an expansion into the enormous areas of the snowy owl’s northern breeding range where there is no monitoring at all — a task that should be undertaken in part by incorporating traditional knowledge of Indigenous communities.

Finally, the study’s authors urged a greater emphasis on salvaging and necropsying dead snowy owls to better understand their health and exposure to environmental contaminants, something Project SNOWstorm has been focused on since 2014.

“Like any long-term project, this research would not be possible without the invaluable contributions from the many individuals who helped with field and lab work and generous funding from all the donors and sponsors,” Dr. McCabe said, noting that SNOWstorm supporters have been central to this effort from the beginning.

The paper is McCabe, R.A., et al. “Status Assessment and Conservation Priorities for a Circumpolar Raptor: The Snowy Owl Bubo scandiacus.” Bird Conservation International 34 (2024): e41.