D+DH In-Depth is our premium, comprehensive corner on America’s No. 1 game animal. In this graduate-level course, we’ll teach you about deer biology, behavior, and ultimately, how to become a better hunter. Want to be the first to get our premium content? Become a D+DH Insider for FREE! 

Countering the ravages of mother nature’s fury on Northern deer herds.

By John J. Ozoga

If white-tailed deer occasionally experience “psychological highs” — and I think they probably do — one such high time for the Northern subspecies must be during the spring break-up, when snow finally melts and gives way to fresh sprouts of nutritious herbaceous forage. And just as healthy spotted fawns play and frolic, so do feisty adult deer cavort after surviving the strict confinement imposed by a harsh winter.

Perhaps such seemingly neurotic behavior has a physiological basis due to the sudden surge of energy-rich food that once again fuels the whitetail’s metabolic furnace to capacity. Maybe that extra energy just naturally allows for frivolous behavior — such as racing about, kicking and jumping into the air for no apparent reason — otherwise not affordable during the depressing months of winter.

To the uninformed, the whitetail’s winter shelter-seeking behavior, commonly referred to as “yarding,” might seem suicidal, as hordes of deer rush to occupy dense conifer stands stripped of browse by previous generations of wintering white-tailed deer. However, many trade-offs — involving nutrition, shelter, conservation of energy and predator risk — influence the whitetail’s bid for winter survival at the northern edge of its geographic range.

The Obligate Migrator

Not all Northern deer migrate long distances from summer to winter range annually. We refer to those that do as “obligate migrators.” Others, referred to as “conditional migrators,” tend to travel shorter distances, and only during occasional winters of extreme severity. Those deer living in the southern Upper Peninsula (UP) of Michigan occupy about 13% of the total UP area, and tend to exhibit conditional wintering habits.

In the northern UP, where winters are consistently severe, most deer migrate from areas of heavy snowfall southward, to areas of conifer cover that typically receive less snow — sometimes traveling 50 miles or more. These traditionally used wintering areas (some used by deer for more than 75 years) are called “deer wintering complexes” (DWCs) and represent only 17% of the landscape. In other words, whitetails in the UP typically vacate 70% of the region during winter because it does not meet their food and shelter needs for survival.

This behaviorism referred to as “yarding” is probably not a deep-seated genetic trait. More likely, it is an adaptation in response to cold weather, deep snow cover and increased predator risk that normally accompany Northern winters. It probably evolved as an energy-conserving and predator defense adaptation for winter survival. Historically, those deer that exhibited yarding behavior survived to reproduce and perpetuate the basic trait, whereas those that did not adapt died.

The best research evidence indicates that deer migratory patterns are learned, not inherited, because some translocated migratory deer switch to being non-migratory while some non-migrators become migratory. Also, some migratory deer continue their established migration patterns, despite being displaced to new range. This suggests that migratory deer exhibit memory for compass bearings and migratory distances and that all deer are born with this capacity to learn migratory patterns.

For obligate migrators, the young animals must depend upon older, experienced individuals for guidance. Associating with older deer helps the younger ones learn lengthy migration routes. Having strong social alliances — including the mother as well as other related adult females — also helps buffer the weaker deer from undue social stress. This can mean the difference between life and death, especially for those experiencing their first, and potentially last, harsh Northern winter.

Death in the Whitetail Wintering Complex

The “walking dead” — severely malnourished animals with irreversible damage to their digestive systems — will leave the confines of their winter homes, too, but with far less exuberance than others that gorge on the lush new growth that spring brings forth.

For those whitetails that surpassed their starvation threshold, the flush of nutritious forage comes too late. Their systems can no longer handle the chores associated with digestion, and there is no hope of recovery. Ironically, the ravages of winter will follow some of them many miles to the bountiful greenery of their summer range, where they might linger a while, then die.

Often, of course, the bones and hides of deer strewn along manure-paved trails within those areas of wintering cover already stand as mute evidence of the stressful winter season. Nearby, ravens perch and squawk, signaling that all is not well in the local deer yards.

Young of the year normally comprise about 80 to 90% of the winter deer mortality during mild or moderately severe winters. However, during a real tough winter — one that starts in late November or early December and carries into late April — adult deer (primarily does) might represent as much as 50% of the loss, most of which can be attributed to malnutrition and/or predation.

Humans might articulate reasons, often weak ones, for excessive winter deer die-offs from one area to another, such as claiming, “This is not good deer country.” And they dwell upon the fact that, “Some deer die every winter, no matter how good the habitat is.”

These same individuals insinuate that not much can be done to reverse the steady downward spiral in Northern deer numbers. I vehemently disagree, because they conveniently ignore the fact that many Northern deer wintering areas, now devoid of deer, once supported thriving deer herds, even during harsh winters.

Northern whitetails seek dense conifer shelter, especially Northern white cedar and hemlock, for comfort, ease of travel and safety from predators. Obviously, however, they cannot survive a prolonged winter on fat reserves alone and must have a favorable food supply in the immediate vicinity.

It’s equally important to recognize that even fat, healthy deer are vulnerable to effective predators such as coyotes and wolves when confined to small, isolated patches of poor cover during times of deep snow.

The message here is perfectly clear: In order to survive harsh Northern winters, whitetails require large areas of habitat that provide good quality protective conifer cover as well as good food sources nearby.

Quality winter shelter consists of Northern white cedar or hemlock dominated stands, over 35 feet tall and growing on good sites with about 70% canopy closure and with a 50:50 ratio of shelter to food.

The Hidden Loss

Pregnant does that venture from these wintering grounds carry with them the next generation — embryos more than half grown when winter lingers into late April — and hold the key to deer abundance in the months and years ahead.

Until spring green-up, the unborn, looking every bit like miniature newborn fawns with spotted coats and floppy ears, have been nurtured primarily by the reserves of their mother’s body. Unfortunately, some of the unborn might already show the signs of malnutrition. Those grossly under-sized late in gestation will not recover sufficiently to survive.

The fate of other unborn fawns is still undetermined. If food, cover and weather conditions are favorable, and there is enough time, prospective mothers might be able to consume enough nutritious forage to meet the fantastic demands of rapidly growing late-term fetuses, not to mention the enormous amount of energy required later to nourish the newborn. Unfortunately, timing of green-up on Northern range is unpredictable and frequently delayed.

Unborn fawns whose mothers face less favorable conditions, not uncommon on Northern range during spring, might not be so fortunate. For them, the cost of inadequate nutrition during the late stages of gestation will result in poor fetal growth as well as other less obvious consequences, and ultimately death.

Well-nourished does tend to produce large, healthy fawns, which they readily care for and tenaciously defend when threatened by predators. On the other hand, malnourished does more often produce stunted fawns weighing less than 5 pounds, which are less likely to be defended and often are abandoned.

Stunted fawns tend to be born alive, but too weak to stand and nurse and die within a day or two. Others, even some of respectable size, will be abandoned and die because their malnourished mothers suffer a resultant hormone imbalance leading to lack of milk secretion and/or the ill effects of a maternal care deficit, and fail to defend them. Fawns so distressed invariably fall victim to predators, thereby artificially inflating predation rate estimates when the real culprit is nutritional shortage.

Boom and Bust

Given adequate shelter and a reasonably good food supply, the healthy Northern whitetail is well-equipped, physiologically and behaviorally, to withstand occasional spells of severe winter weather. Sometimes, however, excessive winter deer kill and subsequent high newborn fawn mortality are unavoidable. The exceptionally tough winter might claim 30% or more of the wintering herd and, later, 50 to 70% of the newborn fawns might die as a result — causing the deer population to crash.

Within the whitetail’s range, however, exceptionally fierce winters occur only periodically, causing deer numbers to plummet temporarily, but then rebound following subsequent easy winters. You can expect Northern deer populations to undergo repeated patterns of boom and bust — this is a natural phenomenon. However, the seriousness of the population decline and amount of bounce-back will hinge heavily upon the amount of good quality deer wintering habitat.

Unfortunately, many historic deer wintering complexes, dominated by conifer cover, no longer support deer even through winters of only modest severity, because they are deficient in food and/or shelter.

It takes longer for deer population recovery where the amount of high quality deer wintering habitat is on the decline, as in Michigan’s UP, for example, where the deer population lows have become lower and so have the highs. Over time this roller-coaster pattern has led to steadily declining deer numbers. In fact, estimates indicate there are fewer deer in Michigan’s UP today than there were during pre-settlement times — despite more than 100 years of deer population and habitat management.

The Blame Game

The specific reasons might vary, but the results are similar across Northern deer range: Deer populations are steadily declining because annual deer recruitment rates fail to keep up with deer mortality rates. This simply means that not enough newborn fawns survive annually to replace those deer that die from all causes (natural as well as human induced).

Since natural deer mortality invariably far exceeds human related losses due to hunter harvest, highway accidents, etc., it’s my contention that an improvement in deer winter habitat is the only way to curb the current trend in declining deer numbers on Northern range. Conversely, saving deer from harvest or improving their summer range will make little difference if the winter habitat cannot support more deer.

In many Northern areas, mismanagement of whitetail populations and/or their habitat have been far more devastating than the effects of periodic severe winter weather. Insufficient deer harvesting or disproportionate harvesting of the sexes has led to deer overabundance as well as herd composition imbalances, overbrowsing and accelerated habitat deterioration.

In other cases, the combination of human encroachment and poor forestry practices — often driven by economic demands, not concerns for deer welfare — have fragmented, failed to regenerate or outright purposely destroyed valuable deer wintering habitat.

So, it’s unwise to make excuses that dwell on the idea of unavoidable deer mortality in order to explain routine massive overwinter losses and chronic reproductive failure of whitetails living in Northern environments. A closer look will invariably reveal that such maladies are more often the product of human error, and are avoidable. In my view, such problems can be corrected with careful planning, wise habitat management and cooperation among all those parties involved.

The Solution

To curb the downward trend in deer numbers throughout the species’ Northern range, we must increase overwinter whitetail survival rates while simultaneously decreasing newborn fawn mortality rates.

In the Northern Great Lakes region, in particular, I believe this can be accomplished via a serious, well-coordinated effort to improve the quantity and quality of food and shelter available to whitetails in conifer stands they currently use, or previously used, during winter.

Until recently, here in Michigan’s UP, there have been no well-coordinated comprehensive management plans to rejuvenate deer wintering complexes, probably because there has been no political pressure to do so.

Some field biologists have called attention to the need for such efforts for years, but have been largely ignored by important people in high places. The Upper Peninsula Habitat Workgroup (UPHWG) was formed by the Natural Resources Commission with a charge of identifying and addressing the decline in overwinter deer habitat in Michigan’s UP. The workgroup is composed of representatives of state and federal agencies, corporate/industry forest representatives, members of the Conservation Districts and UP sportsmen’s clubs. Biologists embarked upon a serious planning program designed to rehabilitate deer wintering habitat across Michigan’s UP.

Understandably, this is complicated stuff, requiring extensive study, careful planning and a great deal of cooperation among all those involved. In most cases, these are large landscapes, some over 200 square miles in size. They involve ownership by state, federal and private forest industry agencies, as well as the general public. Hence, proposed management will be complicated by a host of contrasting interests, opinions and goals.

To date, more than 40 such DWCs have been identified and mapped with land cover types identified by ownership. The strategic objective is to work toward and maintain an approximate 50% food and 50% shelter ratio within the DWC. Emphasis will be placed on identifying existing deficiencies in deer shelter and/or food and determine management actions and locations to address the imbalance.

Primary shelter species are Northern white cedar and hemlock, with secondary conifer species including white spruce, balsam fir and white pine. Important food sources include aspen and hardwood browse, as well as oak mast and herbaceous plants in forest openings within the wintering complexes. Forest openings are the first to lose snow cover in spring and provide energy-rich food sources critically important for pregnant does entering late stages of gestation. Logging, generally, will be restricted to the winter months to provide availability of felled browse.

Specific management strategies will vary somewhat from one wintering complex to another, depending upon a host of complicating environmental factors and human interests.

Application of proper silvicultural procedures will be determined by state, federal and private foresters and biologists. The primary goal is to provide high-quality shelter and abundant food sources, distributed throughout each complex, to satisfy the metabolic needs and sedentary (energy-conserving) nature of wintering whitetails, while simultaneously providing them safety from predators.

Conclusions

I’ve spent over 60 years hunting, studying and writing about white-tailed deer. Given my experience, I’m admittedly biased in my views concerning the needs and limitations currently confronting Northern whitetails. More importantly, as a researcher, I’ve come to appreciate this amazing critter’s behavioral flexibility and unpredictable nature. I also realize that many professional wildlifers and foresters have had grossly different lifetime experiences from my own, and are equally biased, some in a totally different direction. Not all will agree with my priorities and assessment of this problem, nor with the solution as proposed here. But none can ignore the current sad status of low deer numbers across their Northern range.

Unfortunately, I’ve heard some say: “With climate change and global warming, deer won’t need protective winter cover in the future, anyway.”

I just can’t accept the idea that Northern whitetails have minimal need for good protective winter cover when confronted with deep snow, even when having an abundance of natural forage. (Availability of supplemental food or energy-rich farm crops is a totally different matter.)

In my view, we have a choice. We can keep the status quo, accept steadily declining deer numbers across Northern deer range (while praying for mild winters) or we can launch a sensible program to more adequately address the Northern whitetail’s current seasonal deficiencies in food and shelter resources and increase the carrying capacity of their winter habitat.

The region-wide program in progress for improving deer wintering habitat in Michigan’s UP is unique. And, it’s scientifically sound. Granted, there will be some heated debate among all of those involved, relative to location, timing, type and intensity of specific practices employed.

Most certainly there will be some trial and error along the way causing specific ideas and procedures to change with experience. It took decades of mismanagement to reach this current predicament and it will take considerable time to see the results of corrective actions.

Progress is encouraging. The overall management plan as discussed here is being well accepted by professionals, deer hunters and the general public. Personally, I’m more optimistic now than ever and confident that we are finally on a potentially productive course of action to increase and stabilize this struggling Northern deer herd at a socially acceptable and more huntable level. Only time will tell.

— John Ozoga has been D&DH’s top research contributor for more than 25 years. He is a retired deer research biologist.

Literature Cited

Doepker, R. V. et.al. 2015. “Characteristics of Whitetails Trapped and Tagged in Michigan’s Upper Peninsula,” 1989-2006. Michigan DNR, Wildlife Division Rept. No. 3605. 52 pages.

Ozoga, J.J. 1995. “Whitetail Winter: Seasons of the Whitetail,” Book 2. Willow Creek Press. Mincqua, WI. 160 pages.

Ozoga, J.J. 2000. “John Ozoga’s Whitetail Intrigue.” Krause Publications Inc., Iola, WI. 206 pages.

UP Habitat Workgroup 2016. The UP Habitat Workgroup Homepage. Retrieved from Michigan DNR website.

Verme, L.J. 1977. “Assessment of Natal Mortality in Upper Michigan Deer.” Journal of Wildlife Management 41:700-708.

Verme, L.J. and J.J. Ozoga. 1971. “Influence of Winter Weather on White-tailed Deer in Upper Michigan.” Pages 16-28 in A. O. Haugen, Ed. Proceedings of the Snow and Ice Symposium. Iowa State University.