Deer are starting to die from winter's deep snow

DULUTH, Minn. ⁠— The blizzards and snowstorms of November and December might seem like eons ago for most of us two-legged creatures, but several months of deep snow on the ground may have catastrophic consequences for many of the Northland’s whitetail deer.

Deer have been struggling through snow two and even three feet deep most of the winter in Northeastern Minnesota, expending critical energy stored in their bodies. Now, that energy is running out before winter has ended, and it’s likely more deer will die as winter drags on in the north woods.

"The tank is running out of gas. The fat reserves are gone,’’ said Tom Rusch, Minnesota Department of Natural Resources wildlife biologist stationed in Tower. "Each day a deer has to endure trudging through chest-deep snow is a significant drain on its physiological condition."

This year it’s been the length of deep snow on the ground, not the cold, that’s caused deer the most grief. (December and January were actually warmer than normal, which probably helped deer some.) While moose have adapted over millennia to deep snow with longer legs, deer, at the northern edge of their range, and relative newcomers to the region over the last century, struggle to find food and shelter when snow is deep.

Wildlife experts say deer mortality due to winter weather and predation will be significantly above average after this winter. Deer will continue to lose fat until spring provides more nutritious food, and that could be well into May.

"Deer mortality will increase daily until spring green-up,’’ Rusch noted.

In a comprehensive, multi-year field study of adult does north of Cook, none of the radio-collared deer had died through this winter — until last week when three of the does perished. "So it’s already starting,’’ Rusch noted of winter’s impact.

Minnesota wildlife biologists have for a half-century been using a simple winter severity index to measure winter’s impact on deer. Every day there is more than 15 inches of snow on the ground, and every night it dips below zero, a single point is awarded.

At the end of an average winter the Tower area will have a winter severity index of about 115. Already this year it’s at 130. And with 15 or more inches of snow expected to remain on the ground throughout most of March, and with a few more below-zero nights possible, that number will likely hit 160 or even 170 by winter’s end.

"That’s pretty severe. We don’t have that many winters on record over 160,’’ Rusch said.

The impact will be most noticeable from Duluth north, with areas to the south seeing fewer days with 15 or more inches of snow on the ground. Duluth still has nearly two feet of snow on the ground. Around Isabella, deer have been wading through deep snow for 95 days already, with the winter severity index at 135. That area still has nearly three feet of snow on the level, meaning winter severity points will likely accumulate into April.

In a normal winter about 10% of deer perish. But in the winter of 1995-96, among the worst on record, with winter severity indices over 200, more than 40% of adult does perished.

"And the adult does are the last ones to die. The fawns go first, then the bucks, then the does,’’ Rusch said.

Severe winter impacts can last for years. Not only do young, old or under-nourished deer perish but surviving does can have fewer or no fawns the following spring because their winter nutrition was poor. Malnourished does often have stillborn fawns (the fawns are not reabsorbed by does as commonly believed) or fawns are born alive but the winter-stressed does are unable to produce enough milk and the newborns die.

"That’s the hard part for many people to understand, but it's a huge impact on the overall population,’’ Rusch said. "You don't notice fawns that were never born alive."

That reduced fawn crop, next year's breeding does and bucks, is what really affects future year's deer populations.

Deer die not only due to malnutrition, but weak deer are easier targets for predators, namely wolves. A lack of quality winter conifer cover also leaves deer more vulnerable. And sudden warm spells don't help, either. The thaw-freeze-thaw cycle has created a crust on the deep snow, allowing wolves, coyotes, bobcats and dogs to run on top while deer, when running, break though, making it impossible to escape.

"That started last weekend when it got warm, and it’s a real game-changer. The whole dynamic changes in the woods, and it changes against the deer,’’ Rusch said.

Another tough winter means no quick rebound for the region's deer herd, and that means another year of conservative harvest goals for the DNR. That means more areas of bucks-only hunting this fall and fewer areas with antlerless deer permits available.

Deer numbers in Northeastern Minnesota, outside of agricultural areas, have been lagging behind peak years since about 2014, with frequent snowy winters repeatedly restricting the population from taking off.

The good news, biologists note, is that it only takes a string of a few mild winters for whitetail numbers to bounce back.