The Effect of Coyotes on Pennsylvania's Deer Herd

Along with bobcats and black bears, coyotes are one of the few remaining predators of white-tailed deer following the extirpation—regional extinction—of cougars and wolves in Pennsylvania. Many deer hunters believe that "coyotes keep down the white-tailed deer population." This has led to many coyotes being killed by deer hunters in the name of "saving" the deer herd, but is this true? No. Coming to this seemingly simply answer requires an understanding of population dynamics—how populations change over time—and the ecology of both species.

Following birth, death is the most significant event in an individual animal's life. What killed an animal is significant to the individual, but on a population level, the individual's death and cause of death is relatively unimportant. All that matters is how many individuals died in a year (deathrate), how many animals were born (birthrate), how many animals immigrate or emigrate to or from an area, and how many animals reach reproductive status (recruitment). In Pennsylvania, the deathrate of adult white-tailed deer tends to be stable and mainly driven by hunting. Recruitment is the number of fawns that mature into adult deer. This determines future populations, as mature deer are the only individuals that breed. In Pennsylvania, a deer that survives the hunting season has a 90% chance of surviving to the next year, so fawn survival is an important driver of deer population dynamics.

In some places, coyotes keep white-tailed deer recruitment rate low, but in Pennsylvania they do not. Predation is more complex than the simple killing of animals. The effects of predation are broken down into two types:  compensatory or additive. Compensatory predation is when predators do not add to the deathrate: the animals that they kill would not have survived the year regardless of predators. Here is an example to better understand this complex topic: in a hypothetical Pennsylvania forest where there are no predators present, two deer die a year from starvation or disease. Then, coyotes enter the forest and eat two deer per year, but deer do not die from any other causes (such as starvation). In this example, predation is compensatory. To make this predation additive, two deer would die (from starvation or other causes) and two additional deer would die from coyotes per year. This would double the annual death rate from two to four deer per year, thus making predation additive.

For the most part, coyotes in Pennsylvania are compensatory predators of deer. This means they do not impact the deer population. The deer they eat probably would not have survived the year. This finding has been demonstrated by several studies, one study looked at the deer populations in the Southeast and East Coast, which had recently been colonized by coyotes. If coyote predation was additive then deer populations would go down, but that is not what the study found. It found that deer populations increased following coyote colonization. Another study in South Carolina removed coyotes from a large area for three years, and the researchers found that the deer population was largely unaffected by removal. Instead of being eaten by coyotes, fawns died from other causes like starvation and disease. A study in Delaware reported that only 45% of monitored fawns were still alive 90 days after birth, but not one of the monitored fawns was killed by predators. Again, fawns died of disease, starvation, and vehicle collisions.

A study on fawn survival in Pennsylvania found areas with and without large predators including coyotes, black bears, and bobcats, had a similar survival rate. The study found that the mother is more important to a fawn's chance of surviving than predators. If she is stressed (by humans or poor habitat), she cannot produce enough nutritious milk to feed her fawn(s). Doe age also has been found to have an impact on fawn survival, older does have higher fawn survival rates. Mature, experienced does are more likely to choose birthing sites with security cover and adequate food nearby. Young inexperienced does typically are left with less secure birthing sites and must travel farther from their fawn(s) to find food.

The Delaware study found that the greatest predictor of fawn survival is weight; fawns that are underweight are prone to higher mortality than heavier fawns. Underweight fawns are more susceptible to cold and rain and therefore more likely to contract disease. An inch of rain in one day doubles the risk of death in an underweight fawn. A heavy fawn is better able to regulate body temperature when wet or chilled. Underweight fawns are also more likely to get caught by predators than those at a healthy weight.

Understanding coyote predation also requires understanding the life history and ecology of coyotes and deer. It is generally accepted that only predators 50 lbs. or more can consistently take down healthy adult deer without the help of outside forces like injury, disease, or deep snow. Thus, only the largest coyotes could consistently prey on adult deer. This does not mean that coyotes do not eat deer. A recent review of the diet of coyotes found that cervids (deer, elk, and moose) were the most common food item in their diet followed by rodents, plants, rabbits, and hares. The review could not differentiate between animals that were hunted or scavenged, and according to the study's author, it is likely that most of the cervids that are eaten were probably scavenged (e.g. roadkill, hunter-killed, or individuals that died from other causes).

Since coyotes struggle to take down healthy adult deer, they mainly predate fawns. But fawns are hard to find, they are camouflaged, almost scentless, and spend most of the day motionless in dense vegetation. If a fawn survives its first 30 days, it is highly likely to survive to adulthood, as it will be able to outrun potential predators.

Deer also avoid predation by "predator swamping". This is when all does give birth around the same time each spring. By doing this, there are so many fawns on the landscape that predators cannot find and eat them all. If a coyote can find and eat one fawn, it will be satiated for a few days giving the rest of the fawns a better chance at surviving the crucial first 30 days. By giving birth in the spring, does are setting themselves up for success as this is the time of year when plants are the most plentiful and nutritious. This not only allows does to produce the most wholesome milk possible, but it also maximizes the potential cover for fawns, so they can better hide from predators.

If the goal of coyote population reduction is to increase deer populations, time and money would be better spent on deer habitat improvements than on coyote control. Proper habitat management increases available food and cover in a forest. Improved habitat conditions not only allow fawns to better escape predation it also reduces stress on does, increasing fawn birthweights and allowing them to produce more nutritious milk. Stress on females can also be reduced by establishing "reserve" areas or sanctuaries on a property where recreation does not occur or is limited. Limiting the use of motor vehicle and high-impact recreation in spring when fawns are especially vulnerable will help their survival.

It is important to remember that even with the best management, deer herd size will always be limited by habitat carrying capacity, and the number of animals an area can support. Proper habitat improvement practices can increase carrying capacity. However, it does not go away, and there will always be a limit to the number of healthy deer an area can support.

References

Alex J. Jensen, Courtney J. Marneweck, John C. Kilgo, David S. Jachowski. 2021. Geographic and Seasonal Patterns in Coyote Diet. Proceedings of the 44^th Annual Southeastern Deer Group Study.

John C. Kilgo, Mark Vukovich, H. Scott Ray, Christopher E. Shaw, Charles Ruth. 2014. Coyote Removal, Understory Cover, and Survival of White‐Tailed Deer Neonates. The Journal of Wildlife Management.

Justin R. Dion. 2018. Neonatal Survival and Spatial Ecology of Adult Female White-Tailed Deer in the Functional Absence of Predators. Thesis at the University of Delaware.

Justin K. Vreeland, Duane R. Diefenbach, and Bret D. Wallingford. 2004. Survival Rates, Mortality Causes, and Habitats of Pennsylvania White-Tailed Deer Fawns. Wildlife Society Bulletin.

Tess M. Gingery, Duane R. Diefenbach, Bret D. Wallingford, Christopher S. Rosenberry. 2018. Landscape-Level Patterns in Fawn Survival Across North America. The Journal of Wildlife Management.

Todd M. Kautz, Jerrod L Belant, Dean E. Beyer Jr., Bronson K. Strickland, Tyler R Petroelje, Rahel Sollmann. 2019. Predator Densities and White-Tailed Deer Fawn Survival. The Journal of Wildlife Management.

Reviewed by David Jackson