Any doubts I had about the effectiveness of spinning-winged duck decoys were erased late one fall afternoon when I gazed at thousands of mallards funneling magically into a harvested South Dakota cornfield.

Friends and I were hunkered nearby at a small wetland with a couple of dozen floating decoys. It was a beautiful decoy spread, but it couldn't compete with the single battery-powered mechanical decoy two hunters had in that cornfield.

They quickly shot their limits of mallards, then departed.

Now -- more than a decade after Minnesota considered banning the contraptions and wildlife managers and hunters debated whether their use would hurt duck populations and violate "fair-chase'' ethics -- spinning-winged duck decoys are ubiquitous.

The controversy has mostly faded.

The Department of Natural Resources this winter will consider removing the last restrictions on the devices, including their prohibition the first two weeks of the duck season and their use on state wildlife lands. Any changes would have to be made by the Legislature.

"It may be time to look at whether the restrictions we have are necessary,'' said Dave Schad, Department of Natural Resources deputy commissioner and an avid waterfowler.

"They've become just another piece of equipment, another option for hunters,'' he said. "They are not the threat that people thought they might be.''

That's a major shift in thought, considering the DNR once led opposition to the devices, which mimic the flash of wings of a landing duck.

Agency officials tried and failed to get the Mississippi Flyway Council to restrict their use. And they asked the Legislature to impose a two-year moratorium on the mechanical decoys in 2001, an idea that eventually died due to opposition from hunters.

"When they first came out, we were concerned whether it would increase harvest and create population issues, and about whether people who didn't use them would be at a disadvantage to those who did,'' Schad said.

Since then, he said, those concerns have diminished. Bag limits and season length ultimately control harvest, he said.

The idea of removing the last restrictions has support.

"We're in favor of getting rid of the spinner limitations,'' said Brad Nylin, executive director of the Minnesota Waterfowl Association.

Do they work?

Studies, including a $78,000 project done in Minnesota for the DNR in 2002 by Louisiana State University, show the spinning decoys work -- at least some of the time on some ducks.

Overall, 71 percent of the 510 ducks killed during the study were shot when the spinning-winged decoys were operating, and 29 percent were killed when they were turned off. Mallards seem to be most susceptible to the tomfoolery.

Researchers also found that mallards were almost three times more likely to approach within shooting range with the spinning-winged decoys.

The study found no evidence they reduced crippling or allowed hunters to harvest relatively more drakes than hens -- long an argument for their use.

"Some days they seem to work, and some days they don't,'' said Nylin, who owns one. "I think it's helped for mallards, and not so much for other species.''

Al Afton is a former Minnesota DNR waterfowl biologist and now an LSU researcher who worked on the robo-duck study.

"Hunters who use them kill more ducks than those who don't, but the real question is, what's been the effect on the duck population?'' he said.

Banding studies of mallards show no long-term impacts, he said. In fact, this year, for the first time since 1999, the North American mallard population exceeded 10 million.

"Personally I don't think they are as effective as when they first came out,'' Afton said.

It's difficult to know how prevalent the decoys are.

The DNR conducted a survey of hunters after the 2010 season and found 27 percent used spinning-wing decoys, up from 10 percent in 2000.

Clearly, not everyone embraces them.

"I don't use 'em,'' said Dave Zentner, 76, of Duluth.

In 2003, Zentner created a Concerned Duck Hunters Panel. Among their recommendations: Outlaw spinning-winged decoys.

"We thought doing things the old-fashioned way made sense instead of using technology to limit-out as soon as possible,'' Zentner said.

Several of the legendary waterfowlers on that panel have since died -- as has the push to outlaw spinning-winged decoys.

Zentner is ambivalent.

"You still have to be where the ducks want to go,'' he said.

Doug Smith • Twitter: @dougsmithstrib