DULUTH — Minnesota Department of Natural Resources wildlife biologists are taking a new look at the impact of winter on deer, especially in the northern reaches of the state, and redefining what a severe winter is for whitetails.
DNR biologists are adjusting the long-used winter severity index that measures the duration of snow depth combined with the duration of cold, and the result will be more winters falling into the severe category.
But even as they make the changes, biologists are noting that the winter severity index, or WSI, is not the only critical factor used to determine winter’s impact on deer, said Barb Keller, the DNR’s big game program leader.
“There are other factors, such as deer density and habitat quality, that can mitigate the impacts of winter that don’t show up in a WSI index,’’ Keller said, adding that the timing of deep snow and cold is huge. “A late spring greenup, for example, can compound the impact of winter. But an early greenup can really mitigate a severe winter a lot.”
Keller said DNR biologists couldn’t find evidence linking the winter severity index for any given year with a specific impact on deer. One winter with an index of 150, where most points accumulated early in winter, might have little impact on deer numbers. But the next 150 winter, with more points in March or even April, could be deadly.
“So we’re saying the WSI is only one factor we use’’ to determine the direction of deer populations and thus determine deer hunting seasons, she said.
Quality food, such as agricultural crop residue where available, also can reduce the impact of an otherwise severe winter, with deer better able to withstand cold and snow if they are eating well. The presence of wolves also has an impact, with wolves more easily able to kill deer in deep-snow winters.
The DNR's winter severity index awards a point for every day with more than 15 inches of snow on the ground and another point for every day with below zero temperatures. Those factors are not changing.
In the past, a winter had to hit 180 to be considered officially severe. Now that’s been lowered to 120 points and above. Winters with totals from 51 to 119 will be considered moderate, and winters at 50 or below will be mild. In the past, winters up to 100 were considered mild and 101 to 179 were moderate.
Based on 36 years of data from winter conditions across deer permit areas in the northern forest region, 25% of winters had values of 50 or less; 50% of winters had values of 51 to 119; and 25% of winters had values of 120 or higher.
Mild so far, but late winter is critical
So far Minnesota's winter of 2020-21 has been very mild, with very few areas reaching 15 inches of snow on the ground and only a handful of days with below zero temperatures. That could change dramatically by April, however, with a cold and snowy March hitting right when deer are at their weakest.
“We’ve had years with almost nothing on the index going into February, but by April we have deer dying in the woods,’’ said Tom Rusch, Tower, Minn.-area wildlife manager for the DNR. “Hunters often want things in black and white, like if the WSI (winter severity index) hits 180, all the deer tip over and die. But it’s never that simple in nature.”
Rusch said that, in addition the the long-standing formal winter severity index, wildlife managers in the snowiest parts of Minnesota keep separate statistics that incorporate extra deep snow, prolonged deep snow, crusted snow or unusually late snow cover into spring — all key factors in figuring how local deer populations survive winter.
"If you watch deer struggle in 30 inches of snow on the ground, you know that has more impact than 15 inches. But the WSI (winter severity index) alone doesn't account for that,'' Rusch said
Deep snow is especially tough on whitetails and is considered the largest factor in winter severity. Whitetails aren't native to this region but moved in after extensive logging and fires a century ago. While native moose, with longer legs, are built for deep snow, deer struggle to wade through snow banks both to look for food and to escape wolves.
Not only do more deer die outright in deep snow winters, starving or eaten by wolves, but the does that survive are in poor shape. Instead of having two fawns they may have only one, or none at all. And when that happens repeatedly in one decade, the region's deer population doesn't have a chance to recover. That leads to fewer deer on the landscape, fewer antlerless hunting permits issued and fewer deer harvested in those areas — a trend that has hit parts of Northeastern Minnesota hard in recent years.
While much of Minnesota has probably not noticed, in far Northeastern Minnesota — namely from Duluth north — a string of deep-snow winters, including five of the past eight and three in a row through last year, have combined to hold deer numbers down. In fact, Northeastern Minnesota's Arrowhead region hadn’t seen a run of that many deep-snow winters since the 1960s and '70s when Minnesota deer numbers were so low the DNR canceled the season entirely in 1971.
The Tower area, Deer Permit Area 176, finished with a WSI of 166 last winter (the long-term average is 115) with 114 days of 15 or more inches of snow on the ground. For many weeks there were 30 or more inches on the ground. The area also had 52 WSI points for days with temperatures below zero.
The good news is that deer populations are highly resilient due to their high reproductive potential. Annual pregnancy rates average 90% for yearlings and are near 100% for does from 2.5 to 15 years old, with more than half of those does having twins. If given 3-4 normal or mild winters in a row, deer have the reproductive capacity to rebound quite rapidly. That’s what happened after severe winters in the mid-1990s dramatically reduced deer numbers. By the early 2000s, after a string of mild winters, the region had its highest-ever deer populations.
For more information on winter’s impact on deer survival go to dnr.state.mn.us/mammals/deer/management/wsi.html.
Minnesota DNR winter severity index for deer
Old: 0-99 = mild; 100 to 179 = moderate; 180+ = severe.
New: 0-50 = mild; 51 to 119 = moderate; 120+ = severe.