The Winter Severity Index (WSI), as it’s called by some state resource agencies, is the benchmark wildlife biologists use to monitor the winter’s impact on white-tailed deer.
The results are reviewed in mid to late March or early April when the winter weather normally begins to break. Then the effects of the winter can be used to determine decisions related to the upcoming fall hunt.
The WSI reflects data gathered by DNR staff and volunteers from Dec. 1 to April 30 and measures daily snow depths and low temperatures. The WSI grows by one point for each day the temperature is below zero, and another point for each day snow depths are at 18 inches, or more. Two points are added per day when both conditions exist. In parts of northern Wisconsin the WSI clock began ticking following two days of snow on Dec. 14-15. The WSI categories are as follows:
• 0 to 50 points – a mild winter with 5% or less of the deer herd dying;
• 50 to 80 points – a moderate winter with 5 to 10% of the deer dying;
• 80 to 100 points – a severe winter with 10 to 15% of the deer dying;
• 100 points, or more, is considered a very severe winter with 20% or more loss to the deer herd. The last WSI very severe winter was recorded during the winter of 2013-14.
“We’re right at the point where we start counting WSI points,” said DNR Wildlife Biologist Greg Kessler, who works out of the Brule office. “The recent snow event added up fast, but I noticed it’s already settled an inch or so. It’s too early in the season to make any determinations, although the count has started sooner than it does most years.”
Winter severity doesn’t only affect the over-winter survival of adult deer, it also can affect spring recruitment. Fawn birthing and survival may decrease as much as 30% the summer following a severe or very severe winter. Although does have a better over-winter survival rate than bucks and fawns, their poor body condition after a severe winter means newborn fawn survival will be lower.
A bad winter does more than just impact deer survival, it also can impact body size, antler development and overall herd health. Heavy snow and cold temperatures can make it difficult for deer to find food, so they burn fat reserves faster in order to survive. When those fat reserves are gone, they begin to burn muscle mass – and that can lead to death.
Normally deer are well adapted to survive winters in the north. Their winter coats have hollow hair, almost like a multi-paned window that helps them conserve body heat. Their metabolism slows during the winter, too, so with the typical decreased winter movement that requires less food, most deer are well equipped to navigate the winter in relatively good condition. This is why March snow storms and below-average temperatures that coincide with their increased metabolism at that time of year can be tough on deer.
Deer are the gold standard when many outdoors addicts begin monitoring the winter’s impact on wildlife, but other wildlife species are also a concern to Kessler.
“We have a strong turkey population up here currently, but they have a hard time foraging in deep snow,” Kessler said. “Fortunately, we had a mild fall with an excellent acorn crop in areas with oaks, so the deer should be going into the winter in good shape. Grouse and snowshoe hares should be fine, too, but I do have concerns on how turkeys will fare.
“Bobcats are another species that may have trouble,” Kessler said. “We have a healthy bobcat population, but we’re at the northern edge of turkey and bobcat range. If you go much further north into Minnesota and Ontario, you’re getting out of turkey habitat and will find more lynx than bobcats.
“I get most of my bobcat complaints in harsh winters with deep snow when bobcats begin to focus on residential areas, especially around bird feeders.”
Bobcats are primarily found in southern states. Their much smaller paws makes it harder for them to handle deep snow, where the large pads on lynx paws are ideally suited for deep snow.
Kessler said it’s too early to begin to be concerned about how the deer will fare this winter, but northern Wisconsin is seeing an earlier than normal start for WSI points to begin adding up.
Jeff Pritzl, the DNR’s statewide deer biologist, echoed Kessler’s comments.
“Most hunters tend to be inherently conservative in being willing to curtail harvest in hopes that it will mean better opportunity in the future,” Pritzl said. “Although that is noble and understandable, it creates a disconnect with understanding how the deer herd is impacted by winter.
“Some may be concerned that fewer deer should have been harvested to maintain a ‘buffer’ for the losses that may occur over the winter. But if the winter is going to be hard enough to kill deer, that ‘buffer’ is only going to cause the available food resources to be spread even more thinly, so it will run (out) before winter ends, and that will likely exacerbate the problem,” he said.
“If deer are going to die, the biggest impact will occur with the young of the year,” Pritzl said. “The next biggest impact will be next year’s fawns that will either not be born or not survive due to poor body condition of the does coming out of winter.”
NR wildlife biologists begin tracking winter severity points after Dec. 1 when snow depths reach 18 inches or more, or when temperatures hit zero or below. This year, snow depth points started accumulating Dec. 15 in northern Wisconsin. Photo by Dave Zeug