This past March, two resource managers from the Carlton County Land Department went into the forest carrying a scientific experiment. It was a plywood box, about 3 feet tall, with a small opening in one corner. They climbed a stepladder and chained the box to the trunk of a red pine about 6 feet off the ground. They screwed a broken branch horizontally to the box, just below the small opening. Next to the box, they fastened a white-tailed deer leg to the pine. On a tree opposite the box, they set up two trail cameras. Then they left.

The curious contraption was a denning box for fishers, a medium-sized, secretive predator in the weasel family, whose population is estimated to have declined by roughly half since 2002 in Minnesota. Scientists hoped the box, one of four they put out in northern Minnesota, would entice a fisher to live inside as a substitute for the tree cavities where they prefer to den, yet which are believed to be less common than 20 years ago.

Almost two months later, their hopes were realized when a camera check showed an April 13 image of a female fisher crawling out of the box opening. It was likely too late in the season for the animal to be denning, but the photo offered hope. Ryan Pennesi, a Carlton County forestry and wildlife technician helping with the field portion of the project, sent the image, with some others, to scientists at the Natural Resources Research Institute, who put together a two-year project to study the nest boxes.

"I was pretty excited," says NRRI fisher researcher Michael Joyce. "We were pretty sure that this was going to work."

If it does, Minnesota scientists will have a new tool to study and possibly help the fisher population recover. Cameras will capture photos of the animals. Glue strips above the box openings will snag hair for DNA analysis. The boxes themselves might serve as key denning sites for females.

Fisher populations are believed to have declined mostly because of habitat loss or change, says DNR research scientist John Erb. One of his studies found that almost all radio-collared females denned in tree cavities, in trunks that averaged 20 inches in diameter. When older upland forest is logged or otherwise disturbed, females lose those crucial den sites and fisher numbers will decline. The younger forests that spring up following disturbances are preferred by bobcats, which also may benefit from milder winters. The cats compete with fishers for food and often kill adult females, leaving kits to die of starvation.

This winter, before female fishers start to give birth in early March, researchers will put up at least 10 more boxes of different sizes. After seeing what works, researchers plan to set up at least 60 more sites this summer. They will put boxes in older forests the fishers prefer and in younger forests where the trees are smaller and cavities are lacking, but where fisher prey is sufficient. This will help them examine whether a lack of den sites is a limiting factor in population growth, and whether their contraption might aid recovery.

"Now, den boxes don't get rid of bobcats," says Erb. "But they can still maybe provide more options for denning."