BRAINERD -  For 17 years, I've been monitoring and maintaining a dozen or so wood duck nesting boxes on my property south of Brainerd. Wood ducks, as well as other cavity-nesting species such as hooded mergansers and kestrels, have successfully raised families in the homes I have provided for them.

During those years, I occasionally had trouble with predators -- mostly raccoons -- invading my nesting boxes. But since I monitor my boxes nearly every day during the spring, I've always been able to keep predation to a minimum.

That is, until a few years ago.

In one three- or four-day period back then, eight nesting boxes were invaded. In all cases, the eggs were destroyed, and in some instances, even the hens were killed.

At the time, my wood duck nesting boxes were placed in trees, and I had made no attempt to deter predators. That, I realized, had to change.

Why the sudden shift in the amount of predator problems?

During past years, my occasional predator troubles could always be linked to raccoons. But in recent years, the fisher, a mink-like animal, has become relatively common in central Minnesota, and I suspected it was the predator responsible for invading my nesting boxes. I have seen several fishers on and near my land.

Despite their name, fishers rarely eat fish. They are agile animals and can climb trees like a red squirrel.

Other predators are at work on my property as well. The most recent to invade the North Country is the opossum. Yes, the 'possum. Whether they are a threat to wood duck nesting boxes has yet to be verified, but it's likely they'll learn to take advantage of an easy meal.

I'm not "anti-predator." After all, they belong in our fields and forests, too. In fact, predators, when kept in check, are a necessary component of a healthy environment. But I refuse to provide them with easy access to a smorgasbord of wood ducks and wood duck eggs.

To predator-proof my nesting boxes, I adopted the plan endorsed by the Wood Duck Society. Check out its website (woodducksociety.com) for complete instructions and patterns for building and placing predator-proof nesting boxes. The society also provides information on where to purchase predator-proofing components.

The society recommends placing boxes on posts and to use a galvanized steel cone to turn back predators that attempt to climb up to the boxes. Contrary to popular belief, higher is not better when it comes to placing wood duck nesting boxes. In fact, according to the Wood Duck Society, some studies have shown they prefer lower-mounted boxes. Another plus to their system: no ladders and no climbing, because boxes can be maintained and monitored with both feet on the ground.

After witnessing the depredation of my nesting boxes, I purchased the necessary parts to make all my existing boxes predator-proof. The total cost was about $25, excluding the nesting box itself. Even though the peak nesting season had passed, I followed the suggestions of the Wood Duck Society and, as a trial, placed one box on a post, complete with the metal cone.

To my surprise, two days later a hen wood duck adopted the house. Ultimately, she successfully hatched a brood, and I was able to photograph the downy ducklings as they left the box. Since then, I have predator-proofed all my nesting boxes.

I continue, however, to have problems with starlings that attempt to build their nests in my wood duck boxes. For some reason, starlings remove most of the wood shavings prior to building their own nest, made primarily of grass. By monitoring my nesting boxes often, I am able to simply remove the starlings' nests at each visit, and eventually the black bandits give up. But it is time-consuming and frustrating.

If you have previously placed wood duck nesting boxes on your property, now is the time to maintain them -- before the ice leaves the lakes and ponds. Remove the eggshells and membranes from last season. Also, replace or add new wood shavings to a depth of 4 or 5 inches. Other birds that may use your nesting boxes include goldeneye ducks, mergansers, kestrels, bluebirds and owls. Wood ducks are surprisingly tolerant of humans and will nest in boxes positioned in or close to civilization.

Placing and monitoring wood duck nesting boxes is rewarding and a great way to get up close and personal with this splendid waterfowl species.

Bill Marchel, an outdoors columnist and photographer, lives near Brainerd.