As with most mammals, white-tailed deer tend to produce more young than can be expected to survive. Although the reasons vary regionally, a high proportion of fawns born each year fail to survive to weaning age.
In some cases, formerly overabundant deer populations have stabilized, if not declined, principally because of recent increases in newborn mortality rates.
By John Ozoga
In effect, some such losses prove beneficial, especially on private land where hunters have limited access or, for whatever reason, don’t harvest enough antlerless deer. It also means some deer population models, formerly based almost entirely on hunting-related mortality, now must incorporate the effects of nonhunting factors.
Meanwhile, wildlife managers are becoming alarmed at the escalating loss of fawns and resultant lowering of annual recruitment rates. In such cases, to maintain stable huntable deer populations, managers must devise ways to reduce excessive natural mortality and/or reduce hunting-related mortality.
While there are no cookbook methods to eliminate fawn mortality, there are certain management practices that can be taken to minimize such loss. Understanding when and why fawns die is essential before embarking on such a management program.
Fawns might die before weaning age from various causes, including malnutrition, predation, accidents, disease and parasites, abandonment/starvation and even birth defects. Depending upon the environment, newborn losses might vary from 10 percent to more than 90 percent.
Even healthy, well-nourished herds suffer some fawn mortality. For example, about 10 percent of the fawns born in my square-mile Cusino deer enclosure died annually even while being supplementally fed year-round. In the absence of predators, most fawn deaths were caused by stillbirths, accidents and abandonment resulting in starvation.
Similar low fawn losses have been recorded in the rich farmland of southern Michigan, central Illinois and southern Minnesota, but have generally been attributed to predation by coyotes and domestic dogs.
Clearly, prenatal malnutrition (directly or indirectly) and predation account for most newborn fawn deaths. However, as I detailed in the June 2010 issue of D&DH (“Maternal Defense: When Is It Worth It?”), it is often difficult to separate the effects of prenatal malnutrition from those of bona fide predation.
The fawn’s size at birth will hinge upon the mother’s nutritional status during the final weeks of pregnancy, and the fawn’s birth size will largely determine its early prospects for survival. Most fawns weighing less than 5 pounds at birth will die within a few days — because they are too weak or because their mothers have no milk and abandon them.
Interestingly, high population density and resultant social stress can also lead to increased abandonment and death of otherwise healthy fawns.
Primary predators of fawns include the bobcat, coyote, gray wolf and black bear. Fawn predation has become a serious concern in Eastern states, largely because of the coyote’s range expansion.
In the North, after severe winters, predation of fawns can be largely compensatory because malnourished does frequently give birth to fawns that would have died anyway. However, fawn predation can also be additive, as might occur in the Southeast, where coyotes more likely kill healthy fawns.
In the presence of effective predators, a 20 percent to 30 percent fawn loss can be expected, even among well-nourished deer living in optimal habitat. Healthy populations can easily absorb such losses. It’s another matter, however, when coyotes kill more than 50 percent of the available fawns before they reach weaning age, as they currently seem to be doing in the Southeast.
Although not normally a problem in the North, certain parasites and diseases can periodically account for heavy fawn losses in the South. For example, until successful eradication programs were implemented, the screwworm fly was a serious parasite of Southern fawns. In fact, some researchers contend the success of the screwworm eradication program contributed to increasing deer numbers in the South.
As is the case of prenatal malnutrition and density stress (which lead to fawn abandonment), the true effects of disease and parasitism are often difficult to distinguish from those of predation because sick fawns are less likely to escape from predators. Also, sick fawns frequently have diarrhea and produce odors that attract predators.
Certainly, predators kill young fawns. And I suspect the problem is serious in some areas. However, I believe predators are often credited with killing fawns when other factors are primarily to blame. I’m particularly suspicious when high death rates among fawns less than one week of age are credited to predators after severe winters.
Killing predators is one way to improve fawn survival rates, at least in the short term. For example, a recent study in Alabama demonstrated that experimental removal of coyotes and bobcats more than doubled annual survival rates. Historically, the same was found in Texas.
Unfortunately, the killing of important predators — such as wolves, bears and bobcats — is often closely regulated by state wildlife agencies; only the coyote can be legally subjected to almost unregulated population reduction. Even so, according to Mississippi State University professor Bruce Leopold, killing off coyotes to improve fawn survival might not be the best management approach.
In a 2005 interview with Quality Whitetails Editor Lindsay Thomas, Leopold warned that killing coyotes to increase newborn fawn survival “is a fight you may not want to get into.” In fact, he contends sometimes it might do more harm than good because disruption of the coyote’s strict dominance hierarchy might actually increase coyote predation of fawns.
“If you hit that [coyote] population, it has the ability to respond very quickly to reduced numbers, and a female may crank out that maximum of 10 to 12 pups in a litter instead of two or three,” Leopold reported. “You can worsen the problem. Now you’ve shifted your population from a few old animals that are regulating themselves, potentially killing each other to maintain dominance, to a population of young, inexperienced animals in greater numbers who may have a greater impact on your deer population. So there’s a proper balance between deer and coyotes that regulates itself. You can easily get an overpopulated deer herd, but it’s difficult to get overpopulated coyotes because they don’t mind killing each other.”
It’s not that trapping and shooting coyotes can’t effectively lower predator numbers and improve fawn survival rates. It can, at least in the short term. And habituated fawn-killing coyotes probably should be eliminated. But coyote removal must be intense and the effort must be continued to be effective in the long-term.
In Leopold’s view, a better approach to minimizing newborn fawn mortality is through proper deer population and habitat management.
Managing Population Density
The whitetail matriarchal social system evolved to take advantage of spotty food and cover resources and to cope with predators.Healthy whitetail populations — those that are nutritionally and socially well-balanced — typically have the highest fawn survival rates.
Conversely, deer harvest strategies that disrupt adaptive deer behavioral patterns — caused by insufficient or excessive cropping among sex and age classes — invariably benefit predators and increase fawn mortality rates.
Insufficient harvest and resultant deer overabundance relative to availability of food and cover can devastate deer habitat and adversely affect deer health and productivity.
In the North, overbrowsing by too many deer during winter commonly leads to food shortage and heavy malnutrition-related deer losses during winter, as well as high newborn fawn losses during spring. In the South, deer overabundance more likely contributes to delayed sexual maturity and lower inutero productivity among female deer.
Regardless of the environment, when deer numbers exceed range carrying capacity, deer health and productivity suffer. Excessive browsing/grazing not only adversely impact mother and fawn nutrition, but can also deplete critical fawn hiding cover — making “hider-phase” fawns more vulnerable to predators.
Nutritional and social stress can also delay and protract the rut; cause young, subordinate does to use inferior fawning habitat; increase fawn abandonment rates and result in slow physical development of fawns — all of which tend to increase young fawn death rates. Stressed deer are also more susceptible to disease and parasites.
Unfortunately, to the untrained eye, overpopulated deer range might not look as devastated as you might expect, because plants of low nutritive quality can quickly replace the more nutritious plants that are removed by deer.
At any rate, an adequate deer harvest — aimed at holding deer population size in balance with available food and cover resources — helps to improve deer physical condition and increases fawn survival.
Managing Population Structure
Unnatural sex and age structure can adversely affect deer reproductive performance just as readily as density stress. However, the importance of social factors in regulating deer reproductive behavior tends to vary regionally.
In the whitetail’s matriarchal society, related does establish fawn-rearing territories adjacent to one another. This form of social organization provides for an orderly occupation of favorable habitat. It also assures that infants imprint on their mothers, minimizes predation of vulnerable young fawns and provides for expansion of the clans range during good times.
When threatened by predators, maternally experienced does exhibit superior fawn-rearing skills, more likely defend their fawns and successfully raise a greater percentage of them.Sometimes these older does even provide support for younger does in defense against predators — a trait especially important in open grassland and agricultural habitat. Therefore, excessive harvesting of prime-age does — the herd’s reproductive machines — should be avoided wherever predation is a serious factor governing newborn fawn survival prospects.
Excessive adult buck harvesting can also adversely affect the timing and length of the rut, at least in the South. In South Carolina, especially late and prolonged ruts were recorded in populations where heavy exploitation of adult bucks resulted in unbalanced adult sex ratios favoring females. Selective harvesting to better balance deer numbers with existing food and cover resources, and to better balance adult sex ratios, significantly advanced and shortened the rut.
Researchers theorize the presence of mature bucks and their signposts stimulate and help synchronize estrus among does. Such a synchronized breeding/birthing schedule means most fawns are born during a relatively short interval of time. This results in “swamping” predators with potential prey, hence minimizing fawn loses due to predation.
So selectively harvesting deer to create natural sex and age balance can minimize fawn loses because of predation and improve annual deer recruitment rates.
The whitetail’s behavior, physiology and nutritional needs change seasonally and, no doubt, vary regionally. Invariably, habitat management geared to provide deer with diverse, high-quality foods and cover will best meet these changing needs and produce the healthiest deer populations with the highest newborn fawn survival rates.
In the North, favorable wintering habitat must provide adequate shelter and a good supply of browse in close proximity. However, specific shelter requirements tend to be quite variable, depending on the amount and duration of snow cover. Unfortunately, managing conifer cover for wintering whitetails can be complex. Logging and regenerating white cedar and hemlock stands, while being used by deer in winter, can be especially difficult and requires the assistance of professional foresters and biologists.
Northern whitetails are well-adapted to endure mid-winter stress; it’s the prolonged winter that can be so devastating. The nutritionally critical period for fetal development is late winter and spring. More than 50 percent of the newborns will probably die when harsh, prolonged winters are followed by poor spring nutrition. Regardless of winter conditions, however, good nutrition during April and May will help minimize nutrition-related mortality.
According to University of Georgia professor Karl Miller, winter/early spring nutrition can even be a significant concern in some areas of the South. He suggests “targeting supplemental plants to provide late-winter forage before greenup of native plants … can provide significant benefits to a deer herd.”
Also, when deer return to their summer range, protein requirements are high for adequate milk production and rapid fawn growth. Therefore, favorable fawn-rearing habitat must provide good nutrition for the mother, as well as good fawn hiding cover.
Generally, the best fawn-rearing habitat will be diverse, with ample forest cover and scattered openings. Availability of agricultural crops might benefit older fawns and help meet the doe’s high nutritional needs associated with milk production. However, intensively farmed land seldom provides suitable hiding cover for fawns threatened by predators.
Even healthy deer populations living free from predators in optimal habitat experience some fawn mortality, and most populations with high reproductive potential can easily sustain 20 percent to 30 percent loss of newborn fawns, annually. Prenatal malnutrition and predation are the chief causes of excessive fawn mortality leading to low annual recruitment rates and decreased hunting opportunity.
Poor nutrition among pregnant does during the final one-third of gestation (April and May) is the primary reason for high newborn fawn death rates on Northern range. Managing habitat to improve food and cover conditions during late winter and spring will minimize such loss.
While killing predators might satisfy one’s ego, and temporarily reduce fawn mortality rates, the best way to counter predator-related fawn losses in the long-term is through proper deer population and habitat management.
Remember, whitetails evolved behaviorisms to exploit spotty food and cover resources and to cope with predators.
— Research Editor John J. Ozoga is a retired wildlife research biologist from Michigan.