Aside from the fall hunting season when half a million hunters are afield in New York State, the most critical months for deer survival are late winter, especially when weather conditions have been harsh for an extended period of time.
Deer rely heavily on their fat reserves to survive in northern climates. They are ruminants similar to cows and have a four chambered stomach that progressively breaks down food as it passes from one chamber to the next. During the warmer months a deer’s diet consists of green, succulent vegetation and that allows them to build up fat levels in their bodies. However during winter that food source is non existent, and deer must rely on browse from trees and shrubs. Their winter diet has considerably less nutrition, and accordingly deer will then be forced to rely on stored fat to survive. The quality of winter habitat varies considerably, and unfortunately much of our winter habitat in southeastern NY is mature forest and/or over-browsed landscape.
Deer store fat in three general locations in their body. The subcutaneous fat, found under the skin is utilized first, followed by the mesentery fat found in the gut cavity, and as a last resource fat stored in the bone marrow is used for survival.
How quickly are fat reserves burned during the winter? It depends on a number of factors related to mobility, cooling and the overall severity of the winter conditions. Snow depth is most critical, especially if deer are forced to wallow through deep snow. Long-lasting deep snows can deplete fat reserves at a much faster rate, especially when deer drag their bellies through the snow for extended periods of time. Low temperatures and wind will also have a draining effect on fat reserves.
Typically young-of-the-year fawns are the first to perish from winter-kill, simply because they never achieve the same level of fat depositions that adult deer have. The old, injured and infirm are typically next to succumb. Surprisingly yearling bucks are often among the next to die because they have a smaller body size and smaller amounts of stored fat because they participated in the fall rut instead of consuming fall foods to add to their fat reserves. In severe winters, prime age does are the last group to die from winter starvation. When the herd suffers wide scale losses in all age classes in especially harsh winters, the deer population will be dramatically reduced in the upcoming fall. If starvation conditions occur for any measurable length of time it is critical for deer biologists to be in the field evaluating the impacts of winter. It is not necessary to specifically count the number dead deer but it is very crucial to know what age classes are being impacted by winter-kill in order for deer management permit quotas to be adjusted accordingly.
There are also other times of the year when deer experience increased levels of mortality, but none as long lasting or dramatic as the impacts from winter-kill. In our area a handful of past winters stand out as really severe winters for deer survival. Specifically the winters of 1976-77, 1987-88, 1992-93 and 2003-04 had extensive winter mortality.
Newborn fawns also have a tough time surviving their first summer. Recent studies in Pennsylvania show that only about half the fawns born in the late spring survive beyond the first eight months of their lives. Predators (black bears, coyotes, bobcats, domestic dogs) played a significant role in fawn mortality. Fortunately, a relatively large number of fawns are born in a brief period of time every year and even with high levels of predation, there is still enough survival to maintain recruitment levels.
Roadkills are yet another form of deer mortality, and there are several peaks of deer/motor vehicle collisions throughout the year. Not surprisingly, there is a distinct spike in motor vehicle collisions in the fall when the rut is underway and there is a similar increase in vehicle collisions once fawns become mobile in late June. Lastly, there can be another peak of deer/motor vehicle collisions when the edges of roadways green up in the early spring.
In spite of the other forms of non-hunting deer mortality, winter kill remains as the most potentially significant cause of non-hunting deer losses. After all of the losses from predation, vehicle collisions and hunting season, winter mortality can reduce a deer population to its lowest level at any time throughout the year.
Mother Nature is always “the low hole in the bucket”.