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It all comes down to lemmings

Once the Snowy Owl pair is formed, the rest of the nesting process may seem like a foregone conclusion – but it’s not so simple. In fact, there have been a number years when we haven’t located a single Snowy Owl nest across our 100 square mile study site.

And the most important determining factor of the Snowy Owl’s breeding success? A small, unassuming rodent called the Brown Lemming.

During successful breeding years, up to 95% of a Snowy Owl’s diet is made up of these small mammals. Yet lemming populations fluctuate widely from season to season and, as a result, so do Snowy Owl nesting rates. Snowy Owls are seemingly so dependent on this food source that in low lemming years, the owls may move on or not nest at all.

These population fluctuations are unpredictable and often dramatic. For example, in 1994 we found no nests. The next year we found 54, our biggest year to date. Why this dramatic boom and bust fluctuation?

According to the Project’s Senior Researcher, Denver Holt, it’s hard to say. “We look at various things, one at a time: parasites, disease, lack of snow, predation, too much rain, thaw and freeze, social strife, but none of them give us a clear answer. So it has to be a combination of factors. But in every single case, the success of Snowy Owls is directly related to lemmings. What’s interesting is how quickly the owls can assess lemming numbers and decide to move-on or stay. They just don’t miss.”

For 28 years, ORI has monitored the populations of both Snowy Owls and Lemmings near Utqiagvik, AK. Despite the dramatic highs and lows of the Snowy Owl population in our study area, the trend line from our data shows a long-term decline.

As seen in the photo above, during a strong lemming year, carcasses will be cached all around the nest. ORI has documented as many as 86 around one nest.

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