Winter Hard on Deer Herd

by Kevin Strauss

Even though last Sunday was the "first day of spring," we could have snow here until late April and that will affect how many fawns and bucks survive our winter season.

The DNR has conducted the "Winter Severity Index" (WSI) for the past 40 years to give us a statistical comparison between winters. The winter severity index (WSI) is measured by combining the number of days below zero with the number of days with 15" or more of snow. The WSI is recorded at DNR Wildlife offices throughout northern Minnesota.
The winter of 1995-96, the toughest on record, was 202 at Tower/Ely and 160 at Eveleth. Last winter the WSI was 152. The most extreme winters start early (December) and run late. Northern St Louis County did not receive heavy snow until mid-January this winter. Research indicates snow depth is the most critical factor for white-tailed deer in Minnesota.

The deep snow and below-zero temperatures have been hard on our northern deer herd, according the the Minnesota Department of Natural Resources. The Department of Natural Resources annual Òwinter severity indexÓ for white-tailed deer in Northeastern Minnesota indicates 2004-2005 winter rated at 146 which is above the long-term average score of 125. Temperatures have been about average, but snow depth is above normal. This was a return to the old fashion "real" winters the northwoods. What has been a good winter for snowmobilers and skiers has also been a challenging winter for deer.

DNR Tower Area Wildlife Manager Tom Rusch expects to see increased deer mortality into the spring. "I think about deer like a furnace," said Rusch. "Their (body) furnaces are full at the beginning of winter and they just keep running down. The fawns are the first ones to run out of gas and the bucks are next because they used up a lot of fuel during the rut."

In general, deer are well adapted to our tough winters. Young deer are genetically adapted to handle about 3 months of deep snow and bitter cold. After that, the fawns start to run out of fat reserves and starve or are killed by wolves and other predators. They are the most expendable part of the herd. Stastically, about half of all fawns die each year of starvation, predation and disease.

So far, deer mortality has been mostly limited to fawns and yearlings. The deep snow favors of predators like wolves who can more easily catch deer who get mired in deep snow. The DNR reports finding wolf kills in most areas. Right now, deer are spending more time on south-facing slopes and other areas with less snow where it is easier to travel and find browse (twig food).

According to Rusch, adult buck mortality increases with a WSI greater than 140. Thus far, he has seen very little adult buck mortality. Adult does are the best prepared to survive harsh winters and suffer the least mortality. This is nature's way to ensure survival of the fittest and rebuild the herd.