TWO HARBORS, Minn. (AP) -- The sound begins as a single muffled whump somewhere off in the woods. You feel it as much as you hear it.
Then another whump, and another and another, in ever-increasing frequency. The distant pounding crescendos until it sounds like someone doing a drum roll on a sleeping bag. And then it suddenly stops.
John Anderson, standing alongside a gravel road in Lake County, nods. That's what he has come for on this morning in early May, to hear male ruffed grouse trying to drum up a mate.
Anderson, 58, of Two Harbors is part of a small army of men and women in Minnesota who gather data for Minnesota's longest-running wildlife survey. Anderson, a consultant in transportation issues, does the survey as a volunteer.
Begun in 1949, the ruffed grouse spring drumming surveys have proven extremely reliable in tracking trends in the grouse population. The survey also is the longest-running ruffed grouse survey in North America.
''A lot of states and provinces are envious of this data string,'' said Bill Berg, a wildlife researcher with the Minnesota Department of Natural Resources in Grand Rapids.
Berg has coordinated Minnesota's drumming survey since 1974. He expanded the count to its present 140 routes statewide, enlisting the help of national forests, Ojibwe bands, community colleges, environmental centers, county land departments, national wildlife refuges and volunteers such as Anderson.
Each spring, surveyors drive their 10-mile routes one time, starting at sunrise sometime in late April or early May. They stop at predetermined points each mile, where they stand quietly and listen for precisely four minutes, then record how many drums they hear.
Male ruffed grouse drum from atop logs, stumps, overturned roots, hummocks or boulders. They clamp their feet down and brace themselves with their tailfeathers, then repeatedly strike the air with their wings. The wingbeats are fast enough to create a momentary vacuum, much as lightning does when it flashes through the sky. The flurry of wingbeats lasts five to eight seconds.
At stop No. 3 on his route, Anderson stands with his clipboard at the road's edge. He slips off his wristwatch and holds it against the clipboard. Then, standing as still as a heron, he listens. The woods are full of sound. Woodpeckers drum. Warblers sing. And, once during the four-minute span, a grouse drums.
Anderson notes the drum, gets back into his gray Buick, and drives on to the next stop.
''I've done this so many years, the car automatically stops,'' he says.
At stop No. 5, the woods ring with sound. A stream murmurs. A dog howls. A rooster crows. A grouse drums. A red squirrel registers its displeasure with Anderson's intrusion. A white-throated sparrow sings its high, sweet song. A woodpecker drums.
''Sometimes, you realize how noisy nature is,'' Anderson says at the end of his four-minute stint.
When he finishes his count at 7:20 a.m., Anderson has tallied an average of about one drum per stop. He'll forward his results to Bob Kirsch, DNR area wildlife manager at Two Harbors.
The DNR will use information tallied by surveyors to let grouse hunters know what to expect in the coming fall.
''It really comes down to public relations,'' Berg says. ''Certainly, it helps resident hunters plan trips. It helps nonresident hunters plan trips.''
The survey is not an estimate of population but an index of population. It shows trends, not precise numbers. But over the years, as the grouse population has risen and fallen in its 10-year cycles, the spring drumming counts have corresponded closely with fall grouse harvests.
For reasons not entirely understood, the grouse population in Minnesota usually peaks in years ending with zero, one or two (such as 1990-92) and then declines. Lows in the cycle usually come in years ending in four or five (such as 1994-95).
While lows in the grouse cycle have been fairly consistent through the years, the peaks haven't been as high as they were in the early 1970s. Increased cutting of aspen has been touted as a boon to species such as ruffed grouse and white-tailed deer. But changes in the way that cutting is done may not be benefiting grouse, Berg said.
''We did bigger clearcuts then (30 years ago),'' Berg said. ''Now we're maintaining more and more snags (old, dead trees), and we're maintaining residuals of aspen. Those are predator hiding places.''
Other factors may contribute to lower peaks in the grouse population, he said. Wolves have pushed out most of the coyotes in the forested region, and fewer coyotes mean more red foxes, which prey on grouse. Berg estimates there are two to three times as many foxes as there used to be.
''There's another side to it, too,'' Berg said, ''with hunting and the increasing use of ATVs (all-terrain vehicles) and more and more forest trails. Twenty or 30 years ago, we had refugia deep in the woods. We couldn't get to them.''
''Now, we have more and more trails open into logging sites, and that goes hand in hand with a better grouse hunter. A lot more hunters have quality and well-trained dogs. That's not to say hunting is a big part of the equation, but it's more than it used to be.''
Results of this year's drumming counts will be available in early June.
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