What is Killing America’s Fawns?

Why wouldn’t we envision such scenes? Trail cameras across the U.S. often snap grip-and-grins of coyotes, bobcats, or black bears with freshly killed fawns. But those pictures seldom tell the full story. Recent research suggests fawn predation is much more about bayonetting the sick and feeble than chasing down the hale and hearty.

In effect, many fawns killed by predators would have died soon anyway of other natural causes, a process biologists call “compensatory mortality.” In a 2018 study, researchers Justin Dion and his professor, Jacob Bowman, at the University of Delaware reported that only 49 (45%) of 109 fawns they monitored in southern Delaware’s Sussex County were still alive 90 days after their birth.

But not one of those Delaware fawns fell to predators. No bobcats or black bears live in the region, and coyotes are rare. Deer, however, remain common. Dion notes that the area’s deer population is “fairly dense” at 50 per square mile. The habitat is a mix of fragmented forest and agricultural row crops.

The researchers documented only one coyote in their study area during the two-year project. The lone sighting wasn’t for lack of effort. Dion monitored predator baits with trail cameras from December to March in 2016 and 2017, totaling 1,186 observation nights. He also placed cameras over corn to bait deer from December to April both years, totaling 2,735 observation nights. The result was the lone coyote photo, but no bobcats, no bears, and six domestic dogs. Red foxes showed up in 157 photos, but none was linked to a dead fawn.

Blaming Predators
Most research, however, doesn’t give predators a pass. Although predators rarely suppress deer herds by themselves, predation is part of life in most habitats. Dion, in fact, reports that over 90% of fawn-survival studies the past 30 years cited predation as the leading fawn killer.

And when Penn State University researchers Tess Gingery and her professor, Duane Diefenbach, analyzed data from fawn-survival research from 1971 through 2018 across the United States, all 29 studies listed predation as the No. 1 fawn killer. Of those studies’ combined 938 fawn deaths, 602 (64%) cited predation as the cause. Of the predation deaths, 105 couldn’t be tied to specific predators, but of the 497 identified “killers,” 364 (73%) were coyotes, 68 (14%) were black bears, and 65 (13%) were bobcats.

Even so, fawn-survival rates in those 29 studies differed little from the 45% survival rate Dion documented in Delaware’s predator-free habitat. Gingery’s analysis found the 3- to 6-month fawn-survival rate in the 29 studies was 41% in forested landscapes, with slightly better survival (less than 5%) for every 10% increase in agricultural cover. Overall, fawns living in agricultural areas fared better than their forest-dwelling counterparts.

None of those findings surprise Jim Heffelfinger, the wildlife science coordinator for Arizona’s Game and Fish Department and a researcher at the University of Arizona.

“The percentage of fawns entering deer populations at the end of summer seldom differs much,” Heffelfinger said. “If the herd is at or above the habitat’s carrying capacity, you lose about half the fawns to starvation or disease. If the herd is below the habitat’s carrying capacity, you still lose about half the fawns, but the cause of death shifts to predators. Researchers have documented that in mule deer and whitetails for decades.”

Bowman, Diefenbach, and Heffelfinger think “predation” usually gets listed as the cause of death because predators leave the most obvious physical evidence. Besides appearing in trail-cam photos, predators leave saliva, bite marks, footprints, claw marks, and chewed bones at kill sites. And by eating or carrying off fawn parts, they remove evidence that might suggest other causes of death.

Pinpointing Cause
Meanwhile, unless researchers find a fawn soon after it dies and rush its carcass to a veterinary laboratory for analysis, it’s difficult to know if it was sick, injured, or malnourished. Human-caused deaths—such as road-kills and mowing or baling hay—seldom make noticeable impacts on fawn numbers. In contrast, with help from the Pennsylvania Animal Diagnostic Laboratory, Dion linked most fawn deaths in his Delaware study to emaciation, disease, or birth defects.

Dion’s success in pinpointing cause-of-death began by capturing and fitting 52 adult does with GPS collars and vaginal implant transmitters. The VITs’ sensors alerted Dion of three events: when a doe was ready to give birth, when she dropped a fawn, and when she moved away from the fawn. Dion then drove to the site, found the fawn, and fitted it with a transmitter-equipped collar to monitor its movements. If the collar signaled death, Dion returned to locate the remains. He took those in good condition to the diagnostic lab for necropsies.

Of 42 dead fawns the lab examined, it cited “emaciation” as the cause in 21 cases. It couldn’t pinpoint the cause in seven cases because the remains were too heavily scavenged. The necropsies cited sepsis for four deaths, pneumonia for three, and drowning, trauma, and other natural illnesses, including Theileria cervi, a tick-borne disease. The lab identified a fox’s tooth puncture in the head of an emaciated fawn, but concluded the fawn had just died or was near death when bitten.

Roughly half of the 42 necropsies (20, or 47%) identified the fawns as less than a week old, and 34 (81%) as less than 30 days old. Earlier studies often overestimated fawn survival because researchers could seldom monitor fawns in their first week of life—when they’re most vulnerable, given their lack of size, strength, and speed. Days-old fawns struggle to outrun predators, fight off illness or birth defects, or endure prolonged wet/cold exposure. Fawns reaching their second week grow stronger, bigger, and faster, which boosts their survival odds.

Standard shoulder-to-shoulder field searches often miss newborn fawns. Dion notes that the average age for traditional “opportunistic” fawn captures is 6 days, meaning these fawns had already survived their life’s most vulnerable week.

“That’s a serious issue with some studies,” Heffelfinger said. “If you’re mostly capturing fawns already a week old, you’re missing roughly half the number that were born. That introduces a lot of bias into mortality rates.”

What Kills Fawns
When researchers in previous studies found emaciated fawns, they often assumed does abandoned their fawns, but Dion’s data dispelled that explanation. Necropsies identified milk in the stomachs of 13 (62%) of the 21 emaciated fawns, and GPS transmitters on the mothers showed they were usually nearby when their fawns died.

Dion and Bowman documented three key factors in fawns surviving their first few weeks. The greatest predictor is weight. Fawns less than 6.6 pounds face more risks than bigger fawns. Their risk increased the more it rained, with an inch of rain in one day doubling the death risk. Heavier fawns can probably regulate their body temperature more easily when wet and chilled. Weights for the Delaware fawns varied little based on the doe’s age, but underweight fawns born to does age 4 and older survived at higher rates.

Myriad factors probably cause most fawn deaths. Underweight fawns made susceptible by cold and rain are likely more prone to a virus or bacteria, too, and more likely to get caught by predators. Dion also documented that mature, experienced does usually choose birthing sites with adequate food nearby, while young does typically traveled farther away from their fawns to find food.

And because ailing fawns often bleat loudly in distress, they likely summon predators to easy meals. A 2011-12 study by M. Colter Chitwood at Fort Bragg in central North Carolina  also found that sibling fawns suffered slightly higher death rates, probably because predators stumbled onto the second fawn when zeroing in on the ailing bawler.

‘Fixing’ Perceived Problems
Unfortunately, when all available evidence points to predators as the driving force in low fawn-survival rates, some landowners and wildlife managers try to fix things with “predator management.” But Bowman said killing coyotes usually treats a symptom of fawn mortality, not its root cause.

Because predation often masks actual problems, efforts to reduce predator numbers through trapping and shooting typically fail. Results vary widely by time and property, but even when removals initially help deer, other coyotes typically move in quickly, necessitating unending, intensive removal efforts. As Professor Craig Harper at the University of Tennessee said, controlling coyotes is like weeding a garden: The moment you forsake the work, weeds (and coyotes) pop back up, strong as ever.

Even so, some herd declines in Southeastern states apparently exceed “compensatory mortality” explanations. Coyotes have recently re-established their populations in the Southeast, and their numbers in some regions might sometimes be large enough to suppress deer numbers through “additive mortality.”

Chitwood’s Fort Bragg study, for example, documented a 14% fawn-survival rate during the first 16 weeks. As expected, survival was lowest the first week, and fawns surviving the first eight weeks usually made it into the population. But of 55 monitored fawns that died during the study, 35 fell to predators (30 to coyotes, five to bobcats), 16 to starvation, and one to a motor vehicle. The cause of the other three deaths wasn’t determined.

Other Southeastern studies involving re-established coyote populations also documented low survival rates: 22% in South Carolina in 2012; 33% near Alabama’s Auburn University in 2007; and 26% at Alabama’s Fort Rucker in 2013.

What to Do?
Meanwhile, wildlife managers see little chance of reducing coyote numbers through hunting or trapping. Dion suggests hunters and landowners should manage habitats to optimize year-round forage so females are well-conditioned for carrying, feeding, and tending fawns. To increase deer numbers, managers might need to reduce antlerless harvests, and encourage hunters to pass up older does. Fawns from older does will more likely survive summer than those from 18- or 30-month-old does. Likewise, to reduce deer numbers, hunters should target older, more productive does.

Either way, Diefenbach, Bowman, and Heffelfinger think it’s time to reduce research into coyote impacts on fawns. “There’s little more to be learned in most situations, unless maybe if you’re dealing with an expanding wolf or black bear population,” Heffelfinger said. “But researching coyote impacts on whitetail fawns? Again? At this point, I don’t see us learning anything new about their predator-prey relationship.”

Diefenbach agreed. “Some questions are valuable and need to be resolved, but for the most part, we need to stop collaring fawns to study coyote predation,” he said. “Maybe with multiple predators on a landscape, you might want to see what happens if you reduce bear numbers, but there’s little we can do to change coyote numbers. On the other hand, we know we can maintain a stable deer population with 20% fawn recruitment if we have 80 to 85% survival in adult females. So, it’s better we direct our efforts there.”

Bowman said one such “special situation” warranting future collared-fawn research might be repeating Delaware’s study sometime during the 2030s, assuming coyotes get established.

“Down the road, when Delaware has the same coyote densities everyone else has, we could repeat the study,” Bowman said. “We might just find we have the same fawn-survival rates with and without coyotes, and that we can’t keep blaming coyotes.”

Feature image via Matt Hansen.