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Witness the Snowy Owl Population Fluctuation
Featured Species: Snowy Owl
Sure, you’ve seen Hedwig the Snowy Owl on screen (actually played by male owls), but have you seen one of these stunners in real life? The winter of 2013/14 was great for sightings, as there was a significant irruption throughout Canada and the U.S. Last winter was a reasonable one for Snowys, but this fall may presage another bumper winter, with early sightings clustered in the upper U.S. midwest and Manitoba. So keep your eyes open in case we have another good winter for these magnificent birds.
Snowy Owls are arctic breeders. Every winter some show up in the south, often juveniles whose plumage is more flecked with brown (females are even more barred). But every four years or so an irruption occurs. Why? Not because they aren’t adapted to the cold winters – their thick down allows them to maintain a body temperature of 37.8-40.0°C when it’s -57.0°C out (brrrr). The classic hypothesis posits a series of cycles. After all, an owl has got to eat. It was thought that the irruptions are linked to crashes in lemming populations (their main food source) further north, which have a boom and bust cycle over three to four years. When lemmings peak in the summer, owls fledge several chicks instead of just one or two. This increase in population pushes young-of-the-year birds south. On top of that, when the lemming population crashes, adult Snowies have to migrate south as well. Some scientists have recently suggested there is more to the story, since Christmas Bird Counts show that the numbers fluctuate irregularly from year to year, and lemming crashes are often more regional than the large-scale geographically synchronous owl migrations. Other factors such as snowfall and extreme temperature conditions may play a role. Some Snowy Owls may also migrate between Russia and Canada!
The mechanisms of lemming population crashes are not completely understood either, but one sure thing is that Disney had it wrong - they don’t commit suicide. Rather, population peaks lead to mass migrations (contains a good video), and the large numbers of lemmings become so focused on getting somewhere that they realize too late that they are accidentally enroute to fall off of stream banks and cliffs. It likely more complicated than this though, with many other factors contributing to the population crashes including infanticide, predation, starvation, disease and global warming.
Here in the south, Snowies like open country with Meadow Voles, so be on the watch for the occasional Snowy on a fencepost, stump or rise of ground. They act remarkably tame, so while you can get quite close, please don’t try – you may stress starving birds, and reduce their chance of survival. Be content with the large number of excellent, on-line close-ups.