Wisconsin's Deer Population Growth Above Average

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For the third year in a row, relatively mild winter weather across the north is expected to result in high level of survival and good reproductive rates in white-tailed deer, according to state wildlife biologists. They estimate that the Wisconsin whitetail population will be between 1.6 and 1.8 million animals this fall.

Each winter Department of Natural Resources wildlife biologists across the north record weather data to establish what is called the Winter Severity Index (WSI). Temperature and snow depth are the main components of the index as they have a direct impact on white-tailed deer over-winter survival and reproductive rates the following spring.

The WSI is calculated by adding points. One point is scored for each day with a minimum temperature of zero degrees Fahrenheit or less and one point for each day with 18 inches or more of snow on the ground. If both conditions exist, a day can total two points. Scores are taken at 32 stations across northern Wisconsin between Dec. 1 and April 30.

An average score of less than 50 is considered mild; 50 to 80 moderate; 80 to 100 severe; and over 100 very severe. None of the 32 recording stations reported severe conditions. Moderate conditions were reported only at Upson in Iron County. This was the third consecutive mild winter noted.

"Region-wide, this winter ranked as mild with an average WSI of 33 compared to the 30-year average of 56," said Keith Warnke, DNR big game ecologist. "These kinds of conditions generally promote above average birth rates and survival in the yearling age class."

"Most of the accumulated index points were low temperature points," said Michele Woodford, the report’s author. "Low temperatures occurred sporadically with the coldest overall month being February. The majority of snow cover points were generated in March with deep snows gone by April when deer are most vulnerable to winter effects."

"During this time of national interest in climate change it is noteworthy that the first 30 years of WSI history produced an average index of 68, while over the most recent 30 years that average has dropped to 56."

WSI is measured only in the DNR's northern deer management region where tough winter conditions can exist and have had significant impacts on deer populations in past seasons. Winters are considered generally too mild to have any significant impact on deer populations in the southern two-thirds of the state.

"There is no doubt that mild winter weather the last few seasons is at least one factor in keeping northern deer herds at high numbers," adds Warnke. "There will be ample deer hunting opportunity across the north and throughout the state in 2007."

"That doesn't mean every stand will have a deer under it," says the biologist, "successful hunters are the ones who do a lot of pre-season scouting – patterning deer in their area – so that when the season rolls around, they know where the deer will be."