Dennis Anderson
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Dennis Anderson

Duck behavior is both predictable and mysterious. Weather often influences when these birds fly and how far. Habitat availability and hunting pressure also play roles in what ducks do and when.

Yet predictions about how these fowl might react to these or other variables are difficult to make, as any hunter knows. Sometimes ducks fly in the rain, sometimes they don’t. Wind can be a factor, and usually is — but not always. Plus, different duck species react differently, even to the same stimuli.

Diving ducks such as redheads and scaup provide one example. Most of these birds migrating south from Canada into Minnesota usually arrive in the state according to a schedule, from about Oct. 20 until Nov. 1, give or take a few days.

Blue-winged teal, by contrast, in most years are long gone by the time major flights of diving ducks enter the state.

Mallards are another kettle of fish altogether. They seem willing to stay in the north deep into the fall, no matter the severity of weather in November and even early December, provided they have food to eat and water in which to dabble.

Bruce Davis, a Department of Natural Resources research specialist in Bemidji, is among those who are trying to understand more about duck behavior. This summer, he’ll oversee the third and final installment of field studies aimed at understanding which factors cause mallards to leave the state in fall, how far these birds fly when they do leave and what habitat types they prefer en route.

“The object of the study is to get data on mallards during fall, when we don’t have a lot of information about them,’’ Davis said. “Quite a bit of study has been done about mallards in spring and early summer, during the breeding cycle. But less is known about what they do in fall.”

Davis’ research began in 2015 in both northern and southern Minnesota, when more than 100 captured mallard hens and juvenile drakes were fitted with tiny electronic backpacks capable of transmitting the birds’ locations to a satellite.

Except …

“All the backpacks failed that first year,’’ Davis said. “The water seals didn’t work. Fortunately, the manufacturer replaced them with new ones, so we were able to restart the study in 2016.”

The backpacks, or “tracking units,” as Davis calls them, weigh about 15 grams, or about 1.5 percent of a mallard’s body weight. They don’t interfere with flight and are affixed by one loop that circles in front of a bird’s neck and another loop that coils behind its wings.

Each transmitter carries Davis’ contact information. Hunters who shoot an outfitted bird, in Minnesota or elsewhere, are asked to return the transmitters to the DNR.

“We get quite a few back, and of those we do, we’re able to reuse about 60 percent,” Davis said. “The others are too beat up to recharge and put on another bird.”

The fall of 2016 was unusually temperate in Minnesota, north to south, with no significant weather events in October. Mallards fitted with transmitters reacted predictably, generally lounging in the state until about the beginning of deer season.

“Eighty percent of birds we tracked were still in the state through November 10,’’ Davis said. “Freeze-up that year occurred about the same time in the northern part of the state as in the south, and 50 percent of the tracked mallards were still here until early December, when they left.”

Such delayed flights confirm what many hunters have experienced during recent falls, which increasingly in Minnesota have featured mild weather. Mallards nesting here seem to hang around for prolonged periods during these autumns, while mallards that migrate into Minnesota from Canada similarly postpone their flights.

As a result, Minnesota’s duck season is sometimes closed while significant numbers of mallards still linger in the state.

“When the bulk of mallards did leave Minnesota in the fall of 2016, they pretty much left at the same time from the northern and southern regions, and most flew directly to southern Illinois, nonstop,” Davis said.

Migrating ducks build up fat reserves to sustain them during such long flights. The fat can be converted to energy fairly quickly, Davis said. But birds that cover such long distances in single flights generally need a day or more to recover before migrating farther.

Weather last fall was significantly different from autumn 2016.

“This gave us a different look at the behavior of birds we marked last year as they left the state,” Davis said. “For one thing, birds in the northern part of Minnesota last fall used the central part of the state as they migrated south, which was a little unexpected. Also, because weather last fall south of, say, Detroit Lakes wasn’t too bad, mallards from the northern part of the state hopscotched their way south, rather than making one long flight out of the state.”

Davis will overlay the mallards’ stopover locations with state habitat maps to determine whether, for example, the migrating ducks prefer to spend more time in crop fields or in wetlands, and if the latter, what size and type.

“Looking at the data, we already know that the 2017 migrating mallards spent quite a bit of time in crop fields at night,” Davis said. “The assumption is they’re taking advantage of these areas when hunters aren’t present.”

This summer, DNR technicians again will fit transmitters onto about 60 mallards, divided between hens and juvenile drakes.

The hope, Davis said, is that he and other researchers ultimately will gain enough insights into the behavior of these birds to aid their management — and perhaps increase their numbers when they return to Minnesota in spring.

Dennis Anderson • danderson@startribune.com