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Written by Don Reid
A snowy owl (Photo Credit: Shutterstock).
Snowy owls are one of the most recognizable of Arctic birds. Their striking white plumage is their visual signature and they are often seen by southern Canadians in winters when the owls migrate far south from their nesting grounds on the Arctic tundra. Migrating owls apparently have too little winter food on the tundra and move south to search for better options in the next best thing to tundra – the open prairie grasslands and farming country of temperate latitudes.
These migrations can involve many individuals, but mostly immature birds fledged from a very successful tundra nesting season the previous summer. These baby booms and subsequent migrations are known as “irruptions,” in contrast to years when many fewer owls migrate. But how do the owls fare during these big breakout years?
WCS Canada biologist Donald Reid is co-author on a new study investigating the survival of these owls. By combining 20 years of telemetry and necropsy (cause of death) data for snowy owls across North America, the paper’s authors find that winter survival during irruption years strongly depends on how many other owls are competing for resources in the south and the individual’s depth of life experience. In fact, mortality was more than four times higher in irruption years than in non-irruption years, but only for immature birds.
These immature owls died disproportionately in early winter. Competition for good landscapes in which to hunt seemed to force many of the immature birds into more marginal places to live, places with less food and more human activity. Another pattern was that mortality was higher in eastern North America, where owl abundance fluctuates more widely between years compared to in core winter regions of the Arctic and Prairies where winter abundance is more stable. This probably reflects the fact that densities of lemmings, the prime summer food for the owls, fluctuates much more widely in the eastern Arctic than in the west.
A male Snowy Owl brings a lemming to his mate at a tundra nest (Photo Credit: Frank Doyle)
Overall, the irruptive “boom” of migrating owls, driven by the amazing pulse of tundra lemmings in some years, is somewhat of a “bust” when it comes to long-term population growth because so many owls do not survive the winter. They are largely a “doomed surplus” that an unstable Arctic ecosystem produces fairly regularly. Fortunately, some of the migrants do make it through to maintain the population.
This pattern demonstrates the complexity of ecological systems. What looks like an unstable and inefficient system – the boom-and-bust cycle of owl migration and death in response to similar booms and busts in lemming populations –is actually sustainable over the longer term, with enough owls surviving the “busts” to keep snowy owls going. But if you do see a snowy owl in a more southern setting, please give it space. That owl needs to focus on the all-important task of finding food during short winter daylight hours.
Researchers attach a GPS radio package to an adult owl with a harness in Qikiqtaruk Territorial Park in north Yukon. (Photo Credit: Polly Madsen)
Raptor biologist Frank Doyle releases an adult Snowy Owl after tagging it with a GPS device (Photo Credit: Alistair Blachford).
Photo credits: Banner | Lila Tauzer © WCS Canada