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Ruffed grouse populations rise and fall, historically in 10-year cycles. No one knows exactly why. The varying availability and composition of aspen catkins might have some effect on the numbers of these birds. Perhaps also the abundance or scarcity of other foods preferred by ruffed grouse plays a role.

This year, in Minnesota, the mystery of fluctuating ruffed grouse populations grew even more mysterious.

Drumming counts recorded by the Department of Natural Resources in spring showed an unprecedented 57 percent increase in the grouse population from 2016. Yet reports in recent months from hunters and other sources indicate the population rise was illusory — and that “ruffies’’ might actually have been fewer in number in Minnesota this fall compared to a year ago.

Did West Nile or another disease depress hatching success or wipe out broods this spring and summer?

No one knows.

What is known, as winter takes hold across the northern third of Minnesota, is that deep (or kind of deep) snow — a rarity here in recent years — is needed to protect these birds from avian predators such as hawks and owls, and to shield them from equally hungry mammalian pillagers, including foxes, bobcats and fishers.

To avoid presenting themselves as easy pickings for these marauders, ruffed grouse burrow into snow — when it’s available — and disappear. This also allows grouse to “thermo-regulate,’’ or stay warm.

The question now is whether the state’s ruffed grouse will have sufficiently deep snow to keep themselves safe and warm this winter. Or whether, lacking snow for warmth and cover, their population might decline even further.

So far, only a few inches of snow are on the ground in many points north. Tons of it needn’t accumulate to help grouse through the cold months. But more is needed than what is currently available.

Meanwhile, snow or no snow, hunting for these birds continues through the end of the year.