A problem faced by wildlife managers during severe winters is public misunderstanding of seasonal adaptation strategies used by deer to survive this time of year.
These strategies, or inherent biological “tools” used by deer to cope with winter, are complex and require professional research and interpretation. For example, a winter of deep snow and cold weather usually leads to a popular feeling that deer starve to death due to a lack of food; this is only partly true as the following explanation will show.
To starve, as defined by the dictionary, means to die from lack of food. Winter deer mortality in northern Minnesota is not due as much to starvation, or lack of food, as it is due to malnutrition, defined as the lack of nourishment-providing food. In other words, winter deaths in northern Minnesota deer due to the effects of weather are primarily caused by a lack of quality food, not from a total lack of food to eat.
This is an important distinction to make as will be discussed in the following comments on the progression of the animals’ “energy balance” as it goes into fall, winter and spring.
To understand this, we must take a look at the concepts of how our northern deer have adapted to winter. Winter preparation begins in late summer when deer feed heavily on very nutritious vegetation. At the same time their metabolism (the rate at which food is burned) decreases and the excess food is stored in the body in the form of fat for later use. Winter for deer begins when snow covers this nutritious food supply and deer switch over to woody browse.
If they have a normal fat load, and if winter is not severe, then most deer will probably survive by utilizing stored body fat and because their metabolism is turned down. You might call this a form of hibernation while on their feet.
In mid March, the metabolic rate begins to increase which means deer need to get out of the wintering areas soon, away from low-quality browse and on to range that will provide them more nutritious spring vegetation. If deep snow cover persists late into spring and prevents access to this new vegetation, some of the deer will not survive due to depleted fat reserves. Under these conditions, most of the deaths will be fawns and not adult does.
Spring then can be a very critical period for some of the deer and it is at this time that supplemental feed may be of some limited benefit. However, the type of feed becomes very important as the problem again is not only a lack of food but also a lack of quality food. It does not help deer to switch from low-quality browse to low-quality hay or grain nor to continue on a diet of low-quality hay or grain. They need a high-quality ration such as deer, rabbit, or horse pellets which are high in protein (at least 12%) and also have the vitamins and minerals which are also so necessary in spring.
It must be remembered that regardless of how mild a given winter may be, some deer are not going to survive. In a more severe winter, more deer (mainly fawns) will not survive. That is simply nature’s way. And, supplemental feeding will not have a significant impact on overwinter mortality.
To rate the severity of the winter and in turn to estimate overwinter mortality, we use an index called the Winter Severity Index (WSI). Two factors are used to compile the WSI; the number of days the low temperature is at or below zero degrees and the number of days the snow depth in open hardwoods is at or over 15”. We begin these readings in November and run through the end of April.
The WSI is used to estimate the overwinter mortality factor for fawns and adult deer. The WSI for any given winter is entered into our deer population model in the computer and this helps us decide how many antlerless permits to issue for the deer season depending upon population goals and other factors. The WSI is not used to determine whether or not deer should be fed, as supplemental feeding will have very little effect on over winter mortality in the forested deer range.