When he laced up his boots last year for the ruffed grouse season, Dan Dessecker’s hopes rode high.

Spring drumming counts in the northern tiers of Minnesota and Wisconsin were compelling and signs pointed toward an off-the-charts harvest. Dessecker struck gold in one of his coverts — triggering flush after flush. But in every other wooded area he walked last season, the birds were missing.

Hunters across the Upper Midwest gave similar accounts. In fact, the great grouse disappearing act of 2017 was so awful, it left a hangover.

Minnesota license sales are down 5 to 8 percent in advance of Saturday’s 2018 ruffed grouse opener and more than 1,000 returning hunters across Minnesota, Wisconsin and Michigan will take part in a pilot study to gauge the prevalence of West Nile virus in the birds. In all three states, grouse is the No. 1 upland game bird species.

Is the disease, which first hit the Upper Midwest some 18 years ago, now striking ruffed grouse with a vengeance? If so, is there anything hunters and wildlife managers can do about it? Or was 2017 just a mysterious blip in the highly cyclical, year-to-year roller coaster of grouse abundance?

Those are the questions that will dominate this year’s hunting discussions.

“We certainly expected a good season last year, and it just evaporated,’’ said Dessecker, a former director of conservation policy for the Ruffed Grouse Society in Wisconsin. “I think it’s fair to scratch one’s head.’’

Charlotte Roy is grouse project leader at the Minnesota Department of Natural Resources. She said last season’s paradox cries out for an explanation. But her personal hunch is that numerous factors played a role in the scarcity, including increasing frequency of heavy rain events in May, June and early July that could be causing high mortality in chicks.

Another theory, borne from data showing a long-term trend of decreasing annual snow cover in northern Minnesota, is that grouse are losing their snowy hideouts from predators. Yet another notion is that something has gone missing from the birds’ diet.

“It could be a buildup of stressors,’’ Roy said. “I suspect it’s not just one thing.’’

The uncertainties are great enough for the Wisconsin Natural Resources Board to consider shortening this year’s grouse season. The season will start Saturday, but it could end Nov. 30 — two months early — if a precautionary grass roots proposal is approved. The idea, scheduled to be decided Sept. 25, is not supported by top Wisconsin DNR biologists.

“Game bird harvest … has little to no effect on game bird populations,’’ said Jaqi Christopher, assistant upland ecologist for the state agency.

Minnesota’s game bird biologists take a similar view and there’s nothing on the table in St. Paul to restrict grouse hunting. But the uncanny shortage of grouse last year stirred an emergency response to ongoing research in Pennsylvania that shows West Nile taking a toll on the birds.

“We decided to do whatever we can as soon as we can,’’ said Ted Dick, an avid grouse and woodcock hunter who is the Minnesota DNR forest game bird coordinator and acting forest habitat team leader in Grand Rapids.

With help from the Ruffed Grouse Society, the Minnesota DNR this week is distributing the remainder of 500 test kits to hunters who will collect blood samples and hearts for the West Nile study. Participants also will record the birds’ age and gender. Observers say it likely will take years of similar data collection in the three states to draw correlations between grouse abundance and West Nile prevalence.

Even if an adverse parallel is proven, Dick and Roy said there’s no obvious remedy.

High priority

In Pennsylvania, the West Nile threat to grouse is stirring action.

A recent Pennsylvania Game Commission report identifies five “high-priority’’ management responses to regional grouse decline. The report calls for targeted and proactive habitat management in response to the state’s own research finding that grouse in areas of high-quality habitat are better able to recover from periods of high exposure to West Nile.

The report urges other states to study their own West Nile dynamics on grouse to better understand where the disease is hitting and how to target habitat improvements.

Grouse populations declined precipitously in Pennsylvania from 2002 to 2005, without much recovery. Researchers there have found a strong inverse correlation between grouse abundance and West Nile prevalence. They also inoculated captive grouse chicks with the disease, confirming that it causes “significant mortality’’ in the birds.

The researchers link West Nile to a troubling trend in their state: Seven of the nine years of lowest flush rates in Pennsylvania over the past 50 years have occurred since 2002. West Nile hit the state’s landscape in 1999.