Deer herd health is subject to weather conditions, and a harsh winter can have a bigger impact than most of us realize.
While there's plenty of variation in range, topographical area, and food sources for North American deer, when it comes to winter, nothing is unaffected by the extreme cold. Some deer mortality is expected during severe winters, but how does it affect overall herd health and well being?
Harsh winters can have an adverse effect on deer herds, but biologists have been studying and watching mortality rates (among other factors) for many years now in the effort to help populations thrive in spite of it. Extreme conditions will have a big effect on the deer population, sometimes for years to come.
Here are some of those influential factors and what they do to deer health over time.
Duration of the Winter
Pregnant does carrying a fawn or fawns have a higher nutritional need than other deer, and to lose these particular animals from the herd can multiply the effect of a winter kill. Early spring can help to alleviate the issues of a deer "eating for two," while a late spring can be detrimental for the chances of survival for both mother and offspring.
More specifically, it's the depth of snow above 12 to 15 inches that has an impact, and depending on how long it lasts, it can be a serious situation.
The deeper the snow the greater effect on deer survival due to cold temperatures, since deer must use more of their energy reserves to move through deeper snow in the attempt to find food.
Back to Back Harsh Winters
One harsh winter can be bad enough, but two in a row can be much worse.
In fact, the Minnesota Department of Natural Resources (DNR) launched a long-term study documenting the winter survival of whitetail deer, including the following year's reproductive success rate. From 1995 through 1996, radio collared deer had a mortality rate of 32 percent of adult does, and across the entire population an overall 40 percent of the deer herd was lost.
Interestingly, the following year (with its repeat severe winter) resulted in adult does having a mortality rate of just about eight percent. The study attributes this to the lack of vulnerable individuals (fawns, old, sick, or injured deer) having been lost the previous winter. As a result, the population density was lower.
When deer endure the winter, they concentrate their numbers in smaller areas and begin to compete for what food is available. As we discussed previously, higher populations create higher mortality rates in areas where the resources are diminishing.
Food and Cover Resources
Deer herds that have access to high-quality forage (such as acorns or other mast) generally enter the winter in better physical condition and have a better chance of surviving severe conditions. In the same way, conifer trees that offer cover are vital for providing shelter from wind and snow.
In areas where the mast is low and where the trees have died or been harvested, deer populations will decrease due to mortality or animal dispersal.
Severe winter weather conditions generally impact fawns the most, followed by mature breeding bucks, and finally adult does. Even if they seem like the strongest of the herd, adult bucks cannot replace their fat reserves due to heavy rutting action, and they are quite susceptible to severe winter weather conditions.
While winter severity and other factors can cause immediate decreases in deer density, whitetail deer have the capacity to reproduce rapidly when conditions allow it. Deer have an excellent winter coat of hollow hair that allows them to conserve body heat and maintain body temperature in harsh conditions.
It would seem that mere exposure to cold is not as difficult on deer as trying to survive deep snow and the food that it covers.
Deer are able to recover from harsh winters, but not before some of their numbers drop from the herd and overall well-being sees a decrease.
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