Winter severity average for area deer

While this week’s snowstorm no doubt added to the overall total, thus far the winter of 2018-19 is being designated as “average” on the Winter Severity Index (WSI) for northern forest white-tailed deer.

How that will all play out come spring is still unknown as we are right around the half-way point of the season but so far northern Minnesota hasn’t experienced any weather out of the ordinary.

According to Tower Area Wildlife Manager Tom Rusch, snow depth and duration of deep snow cover are the two most important factors when it comes to deer winter survival.

“Deep snow and extreme cold are typically factors from mid-December to early April in a normal winter northern St. Louis County winter, averaging 110-120 days,” Rusch said.

The WSI is a tool the DNR uses to measure the effect cold temperatures and deep snow has on the deer population.

In Minnesota, the WSI is calculated by accumulating a point for each day the temperature is zero or below and a point for each day there is 15 or more inches of snow on the ground.

Under 100 is considered a mild winter; 125 is about average for the Tower area; and over 180 is considered severe.

The most devastating winters for deer are the ones that start early and last too long.

Currently, the WSI index ranges from 41 to 60 in the nine local deer permit areas. Rusch said those are pretty typical WSI indices for a northeastern Minnesota winter.

By comparison, the WSI ranged from 27 to 52 at the same time last winter.

“Snow depth in the Tower Area exceeds the 15-inch threshold across the area, ranging from 16 to 30 inches and the majority of northern St Louis County ranges from 16 to 22 inches,” Rusch said. “Northern Lake County has the heaviest snowpack with 20 to 30 inches.”

His comments came Tuesday before much of the area such 10 to 12 inches of new snow.

Rusch added that after the late January deep freeze, deer are utilizing heavy conifer cover. As the season changes, patterns change as well, and the longer and harder the winter gets, the more dramatic the effect on the herd.

During a normal winter, with a WSI score less than 100, deer show normal behavior and are dispersed across the winter Range, Rusch outlined in a memo on winter challenges.

When the WSI climbs into the 101 to 140 range, deer start to show less willingness to run and tend to cluster in high quality conifer and only move on heavy trails, using snowmobile and plowed roads when they can.

When winter reaches the moderately severe level, with a WSI between 140 and 180, the deer start to appear fuzzy-faced, pretty much stick to the well-worn trails or roads, and starvation starts to be a factor.

At the same time wolves will consume less of a deer, preferring only certain parts that aren’t tainted.

During a severe winter, where scores reach 180 to 210, wolves will only kill for surplus and often times dead dear are found untouched by predators or scavengers. Live deer will be found unwilling to run or try to escape and there is heavy starvation and predation.

“Deeper snow is the biggest issue. Mobility is reduced, especially for fawns. Trail networks are reduced. Deep snow has a cumulative impact on body condition and predator escape options,” Rusch said. “The biggest losses are at the end of winter when deer hit the wall. Wolf predation takes its biggest toll in April during deep snow winters. We had 30 to 40 inches of snow by the tail-end of the winter of 2012-13 in the north end of St Louis County.”

Rusch added that the best-case scenario for deer is an early spring. Some winters break in mid-March and by mid-April the fields and roadsides are greening up.

“In an average winter we see winter receding in early April,” he said.


Last weekend while snowmobiling a spotted a real nice buck and with his antlers still attached so I asked Rusch when male deer usually lose their rack.

He said antler casting is dependent the deer’s “nutritional plane,” testosterone levels and winter (stress).

“Northern deer feeding on browse typically shed in January and February. Deer stressed by winter - deep snow and brutal cold - will drop earlier and vice versa in mild winters, Rusch said. “For example, many bucks in Iowa hold antlers into March and early April. Bucks on a corn diet in northern Minnesota can carry antlers into March. Bucks that are stressed by injury or malnutrition will lose their antlers in December.”


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