LE CENTER, MINN. – On Thursday while aviators weighed carefully the reliability of the Boeing 737 Max, Arnold Krueger considered the comings and goings of a different kind of flight.
He had 58 wood duck boxes in his yard and wondered on this late March day when ever-larger flocks of these short-winged fowl would descend on his property to begin anew their nesting seasons.
Krueger, 90, lives a dozen or so miles from Le Center on a spread he and his late wife, Erlys, cultivated for wildlife. Strewn with oaks and interspersed with wetlands and shallow lakes, the acreage is a haven for deer, turkeys, ducks and Canada geese.
Of these, wood ducks receive the bulk of Krueger’s attention, not least now in early spring when these birds are among the first waterfowl migrators to arrive in Minnesota.
“So far I’ve found a single egg that must have been laid Wednesday,’’ Krueger said. “I also found a dead squirrel in one house, and there were squirrel nests in three more houses.’’
About two-thirds of wood duck hens Krueger welcomes each spring are returnees from the year before.
He knows this because he and a friend, Larry Thomforde of Zumbrota, hold a U.S. Fish and Wildlife Service permit to study the birds. Part of their research involves banding hens that nest in Krueger’s boxes so their travels can be documented.
“This morning [Thursday] I saw three hens flying in my yard, and those were about the first I’ve seen this year,’’ Krueger said.
As he spoke, a dozen or so honking Canada geese waddled nearby on the edge of a small lake, pecking among dead grasses for food scraps. Unseen but in the neighborhood as well were hooded mergansers, a less desirable species that also migrates early in spring.
The mergansers sometimes squat in wood duck boxes, laying and incubating their eggs there. They might also “dump’’ eggs in the boxes to be incubated by hen wood ducks.
The opposite also can occur, Krueger said. Mergansers sometimes incubate wood duck eggs.
“One time,’’ Krueger said, “I saw two wood-duck ducklings hatch in one of my boxes with a brood of mergansers. They all got down from the box and walked toward the lake. But when they got to the lake’s edge, the merganser hen shook the wood duck ducklings, intentionally killing them.’’
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Unknown is how many tens of thousands of wood duck boxes inhabit the shores and shallows of Minnesota wetlands and small lakes.
Whatever the number, box maintenance is important at this time of year. Debris should be removed and fresh cedar or other shavings added to ensure hens have the greatest possible chance to successfully bring off broods.
Long gone are days when American forests full of aging trees provided sufficient numbers of natural nesting cavities for wood ducks. Populations today are instead extremely dependent on man-made boxes.
The effectiveness of wood duck boxes was first tested in 1937 by federal waterfowl managers who placed about 700 of the structures along southern Illinois waterways. The late Art Hawkins of Minnesota, along with fellow researcher Frank Belrose, determined the boxes did in fact enhance wood duck propagation.
Made of slab bark, with outsized entry holes, the earliest boxes unfortunately invited disproportionate numbers of egg losses to predators ranging from snakes in southern states to raccoons in the North.
In the years since, studies have resulted in many box improvements, including to the size of entry holes, which generally should be 3 inches high by 4 inches wide and oval-shaped.
In Krueger and Thomforde’s study, the average clutch size of wood duck hens nesting on Krueger’s property has been 15, with 80 percent of eggs hatching.
In 2016, wood duck hens using Krueger’s boxes laid 499 eggs, with 415 hatching successfully, a finding Krueger uses to argue that individuals can in fact positively influence local wood duck populations.
But box maintenance is important, especially now, in spring.
On Thursday, Krueger used a hand rake to remove debris and wet shavings from boxes in his yard, replacing them with fresh, dry shavings.
“In most cases, hens will lay an egg a day once they start laying, and they tend to cover up the first eggs with shavings, perhaps because of the cold weather,’’ he said. “So you want the shavings to be dry enough for her to use until she starts covering the eggs with her own down.’’
When an entire clutch is laid, incubation begins. This ensures all of the ducklings will hatch more or less at the same time.
Wood duck hens failing to bring off broods might renest. And sometimes, two hens will use the same box successively in the same nesting season.
So it’s important to keep boxes clean after hatching has occurred, or should have occurred.
“Every day now, more wood ducks will arrive,’’ Krueger said. “They’re here as soon as water appears along the shoreline of a lake or wetland.’’