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Ruffed grouse follow predictable 10-year cycle; the question is why

Just as they've done every decade for as long as biologists have kept track, ruffed grouse numbers are moving toward a peak in their 10-year "boom-and-bust" population cycle.

Ruffed grouse
Male ruffed grouse make a distinct "drumming" sound by rapidly beating their wings. (North Dakota Game and Fish Department photo)
Contributed/North Dakota Game and Fish Department

Just as they've done every decade for as long as biologists have kept track, ruffed grouse numbers are moving toward a peak in their 10-year "boom-and-bust" population cycle.

But no one, not even the experts, can definitively explain why that happens.

Proof of the trend was revealed again this week when the Minnesota Department of Natural Resources released the results of its spring drumming count surveys. Statewide counts were up 43 percent from 2008.

Here's how the survey works:

Each spring, male ruffed grouse set up shop on drumming logs throughout the Northwoods, rapidly beating their wings to mark their territory and advertise for a mate. The resulting sound resonates throughout the forest, and wildlife managers listen for the drumming grouse along a defined series of survey routes.

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The number of drums they hear at each stop offers an insight into the size of the population.

This year's survey produced an average of 2.0 drums per stop statewide, compared with 1.4 drums per stop last year. The DNR divides the state into four ruffed grouse zones -- Northwest, Northeast, Central Hardwoods and Southeast -- classifying the regions by habitat type rather than geographical boundaries.

The most substantial increase this spring was recorded in northwestern Minnesota, where counts rose a whopping 117 percent -- from 0.9 drums per stop in 2008 to 1.9 drums per stop this year. The Northeastern survey region that comprises the heart of ruffed grouse range rose 44 percent, jumping to 2.4 drums per stop this spring.

The Northeastern survey region extends from central Roseau County east to Lake Superior and south to about Pine County.

The big jump

According to Mike Larson, DNR research scientist and grouse biologist in Grand Rapids, Minn., ruffed grouse populations have been on a steady rise since 2005. But this year's counts, he said, produced the substantial increase managers were hoping to see.

"The last two times around, there was one year where the jump was substantial, and we hadn't experienced that during this current increase phase yet," Larson said. "Last year, we were three to four years out of the low end of the cycle, but the drum count indexes were only about halfway to their peak."

The last two peaks in the cycle occurred in 1989 and 1998, and the most recent bottom in ruffed grouse counts occurred in 2004, when the statewide survey produced only .8 drums per stop -- one-third of this year's tally.

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This year suggests the numbers are catching up with the cycle.

"Drumming counts this year are as high as counts during recent peaks in the population," Larson said.

But at the same time, he cautions, drum counts are not directly related to grouse abundance in a given year. In the Northwest, for example, this year's 117 percent increase in drumming counts does not mean there are 117 percent more grouse than there were last year.

Biologists for decades have tried to figure out what causes the cycle to fluctuate in such predictable 10-year intervals. In one of the most extensive studies to date, Larson said a University of Minnesota study examined grouse populations in an area near Grand Rapids, looking at everything from weather data to the abundance of raptors such as hawks that prey on grouse.

Among the findings: Cold winters with little snow tended to result in lower populations the next year, while warm winters with lots of snow generally meant more birds the next year.

Still, Larson said, the predictability of ruffed grouse cycles largely remains unexplained.

"They looked at any way you can hypothesize and think of that would affect ruffed grouse populations and affect the cycle," Larson said. "The best set of predictors they could pull together explained 17 percent of the variability in the cycle. The other 83 percent remains unexplained, and there's nothing we know about that would explain that part."

Variable trends

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Years of research also has shown that no two cycles are the same. During the peak in the late 1980s, Larson said, grouse numbers took a big jump from 1988 to 1989 and then dropped right back down in 1990. In the late 1990s, by comparison, grouse populations remained near the peak for three years.

"The name of the game is variability," Larson said. "No two peaks are the same. We're in the fourth year of the increasing phase of the cycle. Looking back, the increase phase has lasted as few as four years or as many as eight. Sometimes, the phase is twice as long as during other cycles. You look at things like that and realize that almost anything could happen from one year to the next."

Generally, there's a strong correlation between spring drumming counts and fall hunting success. But there are occasional exceptions, such as 2007, when hunting success took a nosedive from the previous year, despite an increase in drumming counts that spring.

Hunters in Minnesota shot only 294,000 grouse in 2007, compared with 417,000 in 2006.

"That lack of correlation between spring and fall has happened before, but it's unusual," Larson said. "It's a rare occurrence. In general, fall harvest is highly correlated with drum counts from the preceding spring."

Minnesota frequently is the nation's top ruffed grouse-producing state. On average, 115,000 hunters shoot 545,000 ruffs each year, and harvests can vary from 1.4 million ruffs during peak years to as few as 150,000 birds during falls of low abundance.

Dokken reports on outdoors. Reach him at (701) 780-1148; (800) 477-6572, ext. 148; or send e-mail to bdokken@gfherald.com .

Ruffed grouse population
Minnesota ruffed grouse drumming counts (spring 2009)

Related Topics: HUNTING
Brad Dokken joined the Herald company in November 1985 as a copy editor for Agweek magazine and has been the Grand Forks Herald's outdoors editor since 1998.

Besides his role as an outdoors writer, Dokken has an extensive background in northwest Minnesota and Canadian border issues and provides occasional coverage on those topics.

Reach him at bdokken@gfherald.com, by phone at (701) 780-1148 or on Twitter at @gfhoutdoor.
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