Hunting the Freeze Line

By T. Edward Nickens

We first had to break a path through 100 yards of solid ice just to get to the blind. We had known the freeze was coming. Like duck hunters everywhere, we had obsessed for days over the weather forecasts. We packed hatchets and an ax ALONG with our decoys and shell boxes, and in the dark we hacked out 10-foot-wide sheets of ice and carefully slid each sheet under the remaining ice. With half-frozen fingers we brushed shards of shattered ice back into the water so they wouldn't sparkle in the coming sunlight. And with less than a minute before shooting light, we trudged circles in the open hole to hold the refreezing ice at bay.

Then, when the first light lit the skies over Lake Mattamuskeet National Wildlife Refuge, we felt like the kings of Hyde County as we lorded over what might have been the only open water along the entire southern shore of the lake. Sweat-soaked but shivering, hunkered low in a stake blind shaded by tall trees, Scott and I counted down the seconds. Wigeon were in the hole before legal light. Some strutted on the ice surrounding the opening. More birds strafed overhead, drawn by the open water, and with our first shots a white-capped wigeon drake skittered like a hockey puck to teeter on the hole's jagged edge, half in the water and half on the ice. More birds wheeled in, drawn to our homemade honey hole, and it was hard not to feel a little like royalty.

For weather-watching hunters who parse every forecast update, hunting the freeze line is the ultimate scoreas long as you're on the right side of the line, of course. Roughly speaking, the freeze line represents the boundary between those places where there is enough open water for a duck to roost and feed and the places where a duck had better not nod off without a solid flight plan for warmer climes. North of the freeze line, the thinking goes, is a waterfowl wasteland of iced-over ponds, cornfields blanketed with snow, and sloughs frozen like hard candy. To the south, open water and duck food await. And the closer you can cut it, the better your chances of a hunting experience for the books.

That's the theory, at least. And it sure panned out that way on that one memorable Lake Mattamuskeet morning.

The working concept of the freeze line as a bellwether for a good hunt is a solid approach, says Dr. Michael Schummer of the State University of New York College of Environmental Science and Forestry. As a post-doc research assistant at Mississippi State University in 2008, Schummer and his supervisor, Dr. Rick Kaminski, became intrigued with mathematical modeling to predict waterfowl migrations. A diehard waterfowl hunter, Schummer knew how useful such a tool might be if he could identify the various triggers that prompt ducks to fly south. Last year his work culminated in a new project, a mashup of meteorological data and waterfowl biology that allows him to forecast migrations for dabbling ducks across eastern North America. Every Sunday night during duck season, Schummer posts a Weekly Duck Migration Forecast on his YouTube channel, including graphs, charts, and video commentary. Each report predicts the rate of change in the relative abundance of mallards, black ducks, pintails, gadwalls, wigeon, shovelers, and green-winged teal across the eastern half of the continent.

He's not relying on sightings and hunter reports. For starters, Schummer calculates what he calls a Weather Severity Index, or WSI, for various regions along the Atlantic and Mississippi Flyways. He uses data from 10 sites across the United States and Canada for his calculationsweather stations as far north as Churchill, Manitoba, and south to Memphis, Tennessee, and Charlotte, North Carolinaand adds in other regional temperature and snow cover information. The formula might sound like a math problem from your worst SAT-cramming nightmares, but it's remarkably straightforward: Take the average daily temperature in Celsius and flip the positive or negative values. A temperature of -3 C would convert to a value of +3, for example, and +4 would convert to -4. Then add that figure + snow depth in inches + number of days below freezing + number of days with snow cover greater than one inch deep.

Recognizing that duck species differ dramatically in their response to cold weather, Schummer plots those ever-changing figures on charts that forecast the relative rate of change for each species at all 10 data sites. For hardy ducks, such as mallards, the threshold for movement seems to be a WSI index of +5. For pintails, the threshold is a WSI of -4. Wigeon tend to hit the road at a WSI of -10.

It all sounds super wonky, perhaps more suited to a college classroom than a duck blind. But here's the kicker: These charts don't tell you where ducks were a few days ago; they tell you how good the chances are for ducks to show up in your area over the coming week. And they're on the Internet, updated weekly.

"Over the years, scientists have looked at everything," Schummer explains. "Moon phases, length of daylight, sunspots, you name it. But when you study the data, what really matters is how cold it is and how long it has been cold."
And snow cover is a tipping point. Smaller ducks such as shovelers and gadwalls are wetland obligatesthey rarely feed in fields and are closely tied to open water. Like small-bodied teal, they can't take a chance on getting caught too far north, so they migrate early.

But brawny field-feeding birds such as mallards and black ducks can store plenty of fat. They'll hunker down through bursts of brutal weather, as long as larger rivers and lakes retain some open water. They'll even wait out the cold for a few days in case snow-covered cornfields thaw. But day by day, their biological patience wanes. Schummer's WSI calculations seek to put a crosshairs on the threshold between ducks sticking it out and heading south.

It's not a slam dunk. "The science is complex, and the process of working through the data is a bit convoluted," Schummer says. But he figures that if you have a 50-50 chance of picking the right day to hunt, and his science is right just 75 percent of the time, you will improve your chances of picking a good hunting day by at least 25 percent.

And a 25 percent better opportunity of having an awesome day is a chance I'll start taking.

I appreciate Schummer's preoccupation with Siberian snow cover and polar vortices. I live in grits-and-collard-greens country, where a deep freeze is our only good shot at big, red-legged mallards. In my neck of the marsh, we monitor the freeze line like kids watching the Santa Tracker on Christmas Eve. If I could rig my cell phone to send a four-alarm fire notification each time the jet stream dipped below the Mason-Dixon line, I'd do it in a snap. I've long known that it makes sense to float-hunt small streams when cold weather locks down adjacent swamps and ponds. And it doesn't take a scientist to figure out that those clear, high-pressure days behind a cold front are days you don't want to miss in a marsh.

But it's also true, as Schummer relates, that a fixation on the freeze line can function as a blinder to hunters. Migrating birds frequently move east and west, searching for accessible food resources and open water even as the freeze line dips south. You can't throw in the towel just because you can count the nighttime low on one hand. "These birds are much more fluid than we give them credit for," he explains. Just like the ducks, you have to find open water.

Or create it. Long before the advent of Ice Eaters, savvy hunters knew that even the smallest patch of open water can suck ducks in when the world sparkles with ice. Three seasons ago, when the weather forecast called for a beaver-pond-freezing Arctic blast, I bolted for my hunting lease late in the afternoon. I barely had time to get to the water before the sun set, but I needed only a few minutes. And a shovel.

Walking out to where the beaver dam crossed the creek channel, I pulled gnawed logs and branches from the top of the structure and stomped a few breaches into the dam's mud cap directly over the stream's main flow. Water gushed over the dam and poured into the creek. I knew the beavers would hear the flowing water and rush to repair the dam. But I also knew they might not get to the job site right away.

In the black dark of the next morning I crept onto the top of the dam. I set a jerk cord with three decoys on the open water created by my makeshift waterfall. The hole in the ice was maybe 10 feet wide and 15 feet long. Not much. But it was enough. The jerk cord held the ice at bay, and I walked out of the frozen swamp with a limit of woodies, a single black duck, and a slightly guilty conscience given my vandalism.

And while I will for sure be nerding out on Schummer's posts and YouTube videos like a Southeastern Conference football fan monitoring the AP Top 20 rankings, there's a downside to overplanning a duck season based on the ebb and flow of the freeze line. That's because one thing hasn't changed about duck hunting and never willall the planning in the world won't put a single duck in the pot. You go when you can. If the green light falls on a day the high temperature doesn't break the 20s, all the better. But you can't shoot them while watching a computer screen.

Just go. That's what my buddy Steve Wilson and I did years ago on my most memorable freeze-line hunt. Overnight lows were predicted to plunge into the low teens, so the open waters on a reservoir near home would skim over by midnight and freeze solid by dawn. We'd scouted a timbered point where we could break ice close to thick cover, but after sunset a wicked wind settled in from the north. It pushed the lake's water toward the southern shore and flooded the timbered point that we had planned to hunt.

By the time we hiked in an hour before shooting light, the wind had calmed and the water had receded from the woods. Left behind was a weird half-acre of inch-thick ice suspended two feet above the water's surface. The ice cracked and snapped and shattered like breaking glass as we built a rough blind of saplings and cut brush, a percussive soundtrack to what we thought might be a busted hunt.

But the ducks weren't put off by the commotion. These were birds that had misjudged the freeze line and were caught in a world of hard water with their flight feathers down. Sitting with our backs against a red-mud bank, Steve and I picked our shots as the birds came in wings set and calendar-perfect. This wasn't our typical pass shoot on wood ducks and whatever else happened to show up in the swamp. Every duck was a mallard, and every duck was a flight bird, red-legged and practically quacking in Qubcois French. I snapped some of my first published hunting photos. I even found an arrowhead in the mud between my wader boots.

A freeze-line hunt doesn't always work out that way. I've been iced out of the winner's circle just as often as I've pulled an ace from the deck. But that was another morning of royal fun, and a hunt fit for a king. And it goes to show there's never a good reason not to go duck hunting.

Weather or not.

Forecasting Duck Migrations

Dr. Michael Schummer's Weekly Duck Migration Forecast, posted on Sunday nights on the waterfowl ecologist's YouTube channel, could wind up breaking the Internet. These detailed, species-specific, play-by-play commentaries on what duck hunters can expect in the upcoming week are downright addictive. Combining data with graphs and charts, Schummer cuts through the hard science with hunting-specific insights gleaned from his own hardcore waterfowling experience. Each episode covers the upcoming week, posted from October through January.

You can visit Schummer's website at https://schummerlab.weebly.com/duck-migration-forecast.html.