Biologists Discover Why Fawn Survival Rate Is Plummeting

The major fawn predator east of the Mississippi River is the coyote. A number of studies have shown coyotes eat fawns — and in some areas they eat a lot of fawns.

Biologists Discover Why Fawn Survival Rate Is Plummeting

Doe to fawn ratios are receiving more scrutiny than at any other time in modern deer management history. Why? Because in some areas, especially the Southeast, studies show huge declines in fawn survival. I can’t ever remember when there were more studies on fawn mortality and the reasons these studies are being done is the potential and growing impacts of predators on fawns.

The big stimulus for this concern is coyotes, but it’s more than just coyotes. For the first time in the past 100 years, we have a myriad of predators in the eastern half of the United States. Black bear numbers continue to climb and they are spreading their range as well. Bears do eat fawns, but at least two studies concluded that black bears were “opportunistic” predators on deer fawns. They do not seek them out, but eat them when they find one. In most areas it appears they are not responsible for more than 10 percent of fawn mortality, but when combined with other predators, it adds up.

Bobcat numbers are also up, and in some states are at an all-time high. An on-going study in northern Michigan noted bobcats took a surprising number of deer, perhaps because of deep snows in winter. In some areas they take more fawns than coyotes, but in most areas they don’t. Still, it all adds up.

Mountain lions have started to pop up in many Midwestern states. I can’t find much in the scientific literature on lion mortality on deer fawns, and in most of the East there aren’t enough lions to have a real impact on fawns. However, things are different in the Big Cypress National Preserve in southern Florida. Because of a genetic bottleneck, mountain lion numbers in that part of Florida were around 20 animals and there was fear that in breeding was a problem. Mountain lions were then brought to that area from the western United States in 1995. This resulted in a population of 20 cats growing to approximately 150 today. Those cats have decimated the wild hog population in that area. They’ve also hammered the raccoons and have now turned to deer. Deer were already suffering because of much needed changes in the hydrology in the Preserve. Those changes have brought more water to the area, covering up summer and fall deer food. Decreasing food availability and increased lion predation is also compounded by a growing number of other predators such as black bear, pythons and alligators. The result is that there is high predation on adult and fawn whitetails. Of 97 adult deer radio collared in this study, 42 died and 31 percent of that mortality was caused by mountain lions.

Although the above-mentioned predators all take some deer fawns, the major fawn predator east of the Mississippi River is the coyote. A number of studies have shown coyotes eat fawns — and in some areas they eat a lot of fawns.

The major method of study is to capture fawns, then put motion or temperature-sensitive radio collars on them. If the collar remains in one area for a certain period of time, or if the body temperature of the fawn drops rapidly, then the researchers rush in to check on the fawn.

In many present-day studies, when the fawn is found dead, DNA swabs are taken from the area of the body where predation occurred, and the researchers get a positive ID on what predator was involved. Neat bit of crime stopper stuff. So when you see a study on fawn mortality and there is a category listed as killed by “unknown predator,” you know DNA testing wasn’t done or unavailable when the study was done.

Back to the methods used in fawn mortality research. The approach of capturing a fawn and collaring showed, in general, fawn mortality from coyotes in the first 16 weeks of life was around 16 percent in the northern part of the country and 46 percent in the South.

For example, a North Carolina study showed only 5 of 27 collared fawns (18.5 percent) made it to 16 weeks. Fifteen of the 22 mortalities were caused by coyotes or bobcats and 55 percent of the mortality occurred the first week of life. (Note that most of these fawns were not captured on the day they were born and that is a critical factor. I’ll explain later).

iStock Coyote pouncing

iStock Coyote pouncing

Two Alabama studies showed a 33 and 31 percent fawn survival out to 16 weeks, which isn’t good. A Georgia study showed a 29 percent survival and a South Carolina study showed a 22 percent survival of fawns. Even worse.

Things aren’t as bad further North, although coyote numbers are increasing there. In Ohio, a relatively recent study showed coyotes were responsible for 19 percent of mortality. Natural causes claimed 22 percent and in many cases most of those in the natural death category died from starvation.

Thus, coyotes are not a huge factor for fawns in Ohio.

An early 1990s study on the eastern shore of Maryland showed 91 percent survival of fawns from one week of age until the beginning of hunting season. In 2000 and 2001, Pennsylvania researchers captured and followed fawns until death in forested and in agriculture habitats. Of 110 fawns captured in farm country, 72 percent were alive after 9 weeks and 53 percent after 34 weeks. But things were a lot different in the forested habitat where 108 fawns were captured and 57 percent were alive after 9 weeks and 38 percent after 34 weeks.

In the farm country the number one culprit was natural causes at 38 percent. These include starvation, hemorrhage, pneumonia, infection, tapeworms, etc. Predation got a relatively low 17 percent of all the fawns that died, while in the forested area, predation got 70 percent of the fawns that died. Natural causes were only 19 percent. Interestingly, in the forested area black bears were the top predator (37 percent of predator mortality). Coyotes caused 32 percent of the predator mortality and bobcats 7 percent. In farm country, coyotes caused 63 percent of mortality while bears got only 13 percent. None were taken by bobcats. Because deer numbers were increasing in both habitats and over half the fawns survived out to 34 weeks, the researchers concluded that coyote and bear predation was not a major factor for deer survival in either habitat.

This past year, a Pennsylvania study showed a 52 percent survival out to 10 weeks. Of all the collared fawns killed, three were eaten by black bears, three by bobcats, one by a coyote, one by a dog, three from unknown predators (DNA results pending), five from natural mortality and one by car. Again, predation does not appear to be a major factor in Pennsylvania, but what about the five that died from natural mortality? Necropsies were done and some had no milk in the rumen, but some did. The researchers do not know why these five apparently healthy fawns died.

Now let’s back up and look at why the approach of capturing and collaring fawns might not give us the total picture relative to fawn predation. To collar the fawns, researchers search an area, find fawns bedded (or see them with their mothers), capture them and collar them. The problem with this approach is that most of the fawns are already one week old. Several recent studies show this approach might underestimate fawn predation.

These studies used a method that allowed fawns to be captured and collared at birth. Researchers did this by capturing adult does in the summer or fall and implanting a vaginal transmitter. When she gives birth to the fawn, she also gives birth to this implant and it signals the researchers to run in and mark the newborn fawn. With this approach, researchers discovered a lot of predation takes place that first week. In fact, the first week is the worst for fawn mortality from predators, especially coyotes. They concluded that all studies done with captured fawns that missed the first week underestimated the total fawn mortality due to coyotes.

Fawn mortality studies can be frustrating and a bit complicated. We know fawn mortality is lower for fawns with healthy mothers. Do healthy mothers have fewer fawns killed by predators?  We don’t know, but we do know fawn mortality is lower when they are born in a high-quality habitat. Such habitat is usually found where deer numbers have not exceeded the carrying capacity. Maybe more ground cover means better fawn hiding cover.

What we now know is that getting that fawn through the first week of life is critical to its survival. The question yet to be answered is how we best do that.


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