When the temperature drops and the snow begins to pile up, it’s only natural to seek shelter, and deer are no exception.
The northern white-tailed deer is all over Vermont, including the state flag, but it isn’t built to withstand the cold like its cervid sibling the moose. Instead, white-tailed deer engage in a practice called wintering, or “yarding,” where they congregate in special areas to protect themselves from the worst of the cold and snow.
Wintering areas are important to the survival of the northern white-tailed deer population; Vermont is close to the northernmost part of its range.
“Historically, and in some parts of Vermont still, winter kills more deer than anything else,” said Nick Fortin, head deer biologist at the Vermont Fish and Wildlife Department. “In a mild winter, not that many deer die. In a really difficult winter, we can lose 30-40% sometimes of our deer population in certain areas, and maybe more than 50% of the fawns that are going through their first winter.”
“In a really difficult winter, we can lose 30-40% sometimes of our deer population in certain areas, and maybe more than 50% of the fawns that are going through their first winter.”Nick Fortin, Vermont Fish and Wildlife Department
A number of factors influence the deer mortality rate during the winter, including the temperature and snow depth. Temperatures below 0 degrees Fahrenheit and snow depth above 18 inches – which severely limits movement – are particularly brutal. The state tracks these two metrics with a Winter Severity Index, which it then uses to predict winter mortality, Fortin said.
Other factors include body composition going into the winter, and how late in the year the snow finally melts and spring begins. The longer it takes to happen, the more strain experienced by the deer, who have used up their fat reserves by that point.
In order to survive these conditions, deer congregate under dense, mature softwood trees like hemlock, balsam fir, red spruce, and white pine. These conifers block wind and snow, protecting deer from the worst of the cold and reducing snow depth, which allows them to move around and feed more easily.
The department tracks wintering areas through a combination of field observations and aerial photography.
“The ideal is you go out there in the winter and there’s deer tracks everywhere, or you see deer, then you know the deer are there,” Fortin said. “If they’re not currently using it, you’re looking for things like really heavy browse [signs of deer feeding] on vegetation; certain species only get browsed when there’s nothing left to eat, and that’s evidence the deer are in there pretty heavy. Sometimes you can still see trails because they’re worn right into the ground.”
The original wintering maps from the 1980s were made with mylar overlays on topographic maps. Now the maps are digital.
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The department has a separate, internal map of all conifer forests in the state for wintering areas that haven’t been confirmed.
“That is what we would call ‘potential deer wintering area,’” Fortin said. “And so, when there’s a development proposed, if it’s potentially — even if we’ve never actually documented deer wintering there — if it looks like it could be [a deer wintering area], then that would trigger a review, and we would actually confirm whether or not there were deer using that.”
Conserving precious land
Deer wintering areas are covered under Act 250, Vermont’s wide-ranging land use and development law, as “necessary wildlife habitat,” and Section 248, which requires approval from the Public Utility Commission for building energy infrastructure.
“Our standard operation is to work to avoid and minimize impact,” said Noel Dodge, regulatory review biologist at the Barre Fish and Wildlife office. “So if we get a plan that proposes impacts, we work with a developer to try and change that plan to minimize or reduce the impacts as much as possible.”
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Generally, that involves shifting things around — finding different spots or reducing the number of units. For larger developments, or particularly important wintering areas, the department seeks conservation easements: legally-binding agreements limiting how land can be used by both current and future owners. These agreements generally reserve an amount of land proportional to the size of the development for deer wintering.
The law only protects areas and projects covered under Act 250 or Section 248, Dodge said. Some land is covered by zoning laws in a few towns, but most wintering areas are unprotected.
While wintering areas make up only 5-15% of a deer’s range, they use that area for more than 25% of the year, and would not survive without it.
“They’re critical habitat for deer. That’s kind of lost a lot now, deer are almost becoming a pest in some areas,” Fortin said. “But believe it or not, if we didn’t have enough good quality deer wintering habitat, we really wouldn’t have many deer at all in Vermont.”
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