– Appropriately, a duck decoy brought Arnold Krueger and Larry Thomforde together. This was in the 1960s, and Thomforde was vacationing in northern Minnesota when he saw a bluebill decoy on a fireplace mantel that, as it happened, was carved by Krueger.

A waterfowl hunter since childhood, Thomforde knew enough about decoys to tell those that are shaped and painted correctly from those that aren’t.

“This decoy was different,” he said, meaning “better.”

Thomforde was recalling this chance encounter with one of Krueger’s decoys the other day while sauntering across Krueger’s southern Minnesota farmyard. The two have been friends now for more than half a century, with ducks and all things duck related their shared passion.

Hunting has been part of this; it started there. But more so now, biology and numbers incite their fascination with these birds — lots and lots of numbers.

Krueger, for example, is a spry and fit 90 years old, and has 58 — fifty-eight — wood duck houses, or boxes, in his yard. Each is numbered, and many are occupied by hen wood ducks that are sitting on, or incubating, clutches of eggs.

In one box, Krueger counts 11 of the fragile ovals. In another, 14. And so on.

Thomforde, meanwhile, a relative youngster at age 82, has in his pocket a jumble of numbered leg bands. The goal in this outing is to capture a sample of the incubating hen wood ducks and affix a leg band to each.

Everything about ducks is mysterious — thus their allure — and the effort here by Krueger and Thomforde is to gain further insights into the migratory and reproductive behaviors of “woodies.”

“After Arnold bought his farm and started putting up wood-duck houses, he told me about some of his observations of these birds, how they would come back in spring and so forth, but he wasn’t writing anything down,” Thomforde said. “He was just watching them.”

A violinist by training, Krueger led the orchestra at Owatonna High School for decades. He also played in the Mankato Symphony. His son, Arnold, played violin for 42 years with the Minnesota Orchestra, and his daughter, Eleda, taught violin. His late wife, Erlys, was a church organist.

All of which helps explain why a music stand occupies center stage in Krueger’s living room, with a violin nearby. “I still practice,” he said. “I played three pieces at church on Sunday.”

Yet whatever Krueger’s enchantment with music, waterfowl beguile him most. He owns duck boats and canoes by the handful, and decoys by the dozens, the latter of which, in many cases, carved by his own hand.

Then also in his yard are the wood duck houses. Attached to trees, outbuildings, his house — they’re everywhere.

“When my wife and I bought this property, I thought it would be best suited to enhance for wood ducks, not mallards, because of its many ponds and trees,” Krueger said. “At first I started placing the boxes in our woods, next to ponds. I put them high in trees. But they didn’t attract the numbers of ducks I had hoped. And I had to carry a ladder around to reach the boxes. So I moved the boxes to my yard, and placed them lower.”

Thomforde, who holds bachelor’s and master’s degrees in biology, and who retired as a Bloomington Kennedy biology teacher before moving with his wife to the family farm in Zumbrota, watched these goings-on with his friend with deep interest. He had already published a scientific paper on the migratory habits of banded mourning doves, and was even more interested in resolving questions surrounding the seasonal comings and goings of waterfowl.

Such as: How many eggs, on average, does a wood-duck hen lay? How many of these eggs hatch successfully? Will a wood duck that is hatched in a box in Krueger’s yard return to the yard? Will hens that bring off broods on Krueger’s property return to the yard to nest again? If they do return, will they come back to the exact same box in which they incubated eggs previously?

For answers, Thomforde and Krueger designed a 10-year study approved by the Department of Natural Resources and the U.S. Fish and Wildlife Service, during which they will place leg bands on hen wood ducks nesting in Krueger’s yard.

• • •

Removing a hen wood duck safely from her nesting box is a two-man operation.

Using a three-sided net whose fourth side is solid except for a hole the size of a wood-duck-box entryway, Thomforde covered the front of a box while Krueger reached in from a side trap door.

Alarmed, the hen exited the box into the waiting net.

“We try to be very quiet during the whole process,” Thomforde said. “That seems to encourage the hens to stay in the boxes after we put them back in.”

Using a needle-nose pliers, Thomforde required only brief moments to attach a band to the captured hen’s leg. While he did, Krueger counted eggs in the box. Then the hen was returned to the box through its entryway.

Preliminary results of the study, now in its eighth year, are encouraging, and lend validity to the widely held belief that nesting boxes, properly designed and maintained, can positively influence wood duck populations.

Consider:

• The average clutch (number of eggs) size of studied hens has been about 15 — nearly 80 percent of which have hatched successfully.

• In 2016, for example, 499 eggs were laid by hens using 33 of the 58 boxes, and 415 of the eggs were successfully hatched.

• Wood duck hens do in fact return to previous nesting locations, and in some instances to the exact boxes where they nested before. Last year 17 of the 32 hens that occupied Krueger’s 58 boxes were repeat visitors.

“We’ve had four or five hens come back four years in a row to the exact same box,” Thomforde said.

If there’s a weakness in the study, Krueger and Thomforde say, it’s the relatively few hens banded. The most they’ve done in a summer is 38, with an average of about 30.

Still, though their findings are preliminary, they are comfortable offering those who tend wood-duck boxes the following advice:

• When building a nesting box, use 10-inch wide (usually cedar) boards, not 12-inch, which are commonly used. “Hens seem to like the smaller boxes,” Krueger said.

• Clean the boxes in spring, and add shavings or other bedding. “You can’t have wasps in there, or starlings,” Thomforde said.

• Predators can be a problem. Devise means to keep them from hens and eggs.

“Before the study, I knew, or believed, that wood ducks, like other ducks, often came back to where they fledged, or in the case of hens, where they nested previously,” Thomforde said. “But to see them come back to the same yard, and in some cases to the exact same box, after migrating to the deep south for the winter is amazing.”

danderson@startribune.com