Deer face threats from every direction: predators, disease, harsh winters, vehicles, hunting, and even pesticides in the water they drink. No single factor dominates across all regions and age groups, but together these pressures shape whether a deer lives a full life or dies in its first year. Understanding what kills deer, and when, reveals how fragile survival can be at every stage.
Predation Hits Fawns the Hardest
For young deer, predators are the single biggest obstacle to survival. A study tracking collared white-tailed deer fawns in northern Georgia found that predation accounted for 82% of all known deaths in the first 12 weeks of life. Coyotes were the top predator, responsible for 40% of fawn deaths, followed by black bears at nearly 22% and bobcats at about 13%.
Adult deer face far less predation pressure, especially in the eastern United States where large predators like wolves and mountain lions have been largely eliminated. In regions where wolves still roam, such as parts of Minnesota, Wisconsin, and the northern Rockies, they remain a significant source of adult deer mortality. But across most of the country, the predation bottleneck happens in the first few months of life, when fawns are small, slow, and rely almost entirely on camouflage to avoid detection.
Disease: Slow Killers and Fast Outbreaks
Two diseases stand out for their impact on deer populations: chronic wasting disease and epizootic hemorrhagic disease. They work in very different ways, but both can devastate local herds.
Chronic Wasting Disease
Chronic wasting disease (CWD) is caused by misfolded proteins called prions that destroy brain tissue over months, eventually killing every infected animal. There is no cure and no vaccine. Prions spread through saliva, urine, feces, and blood, and they persist in soil for years, meaning a contaminated area stays dangerous long after sick deer are gone. As of 2025, CWD has been detected in at least 36 U.S. states and five Canadian provinces. The overall infection rate among free-ranging deer remains low nationwide, but in areas where the disease has circulated for years or decades, local infection rates climb much higher, gradually eroding the herd from within.
Epizootic Hemorrhagic Disease
Epizootic hemorrhagic disease (EHD) kills fast. Spread by biting midges rather than deer-to-deer contact, EHD outbreaks erupt in late summer and early fall, typically between August and October, and can reduce a local deer population drastically in a matter of weeks. The disease carries a very high mortality rate, and because outbreaks are concentrated geographically, they can wipe out deer in a specific area while neighboring populations remain untouched. The saving grace is seasonal: outbreaks end within about two weeks of the first hard frost, which kills the midges that transmit the virus.
Winter Severity and the Starvation Window
Winter is the most predictable annual bottleneck for deer in northern climates. Researchers measure winter severity using a cumulative index that assigns points for each day the temperature drops below roughly minus 18°C (0°F) and each day snow depth exceeds about 38 centimeters (15 inches). When that index climbs high, deer start dying.
During one severe winter in Minnesota with an index score of 199, adult female deer experienced a 46% mortality rate. In Ontario, researchers predicted that deer populations had less than a 10% chance of persisting in areas where the average winter severity index reached 167 or higher. Each unit increase in winter severity raised a deer’s mortality risk by about 1.9%, while each additional snow-free day in late winter and spring reduced it by 8.2%. Body weight matters too: every extra kilogram of mass heading into winter lowered mortality risk by 4.3%, which is why deer that enter the cold season in poor condition are often the first to die.
Deep snow is especially dangerous because it buries the low-growing browse deer depend on and makes movement exhausting. Deer burn more calories wading through snow than they can replace from the limited food available, creating an energy deficit that compounds over weeks until their fat reserves run out.
Parasites That Drain Energy Reserves
Winter ticks add another layer of stress during the cold months. These parasites attach in the fall and feed through winter and spring, and heavy infestations can drain enough blood to cause severe anemia. While winter tick die-offs are best documented in moose, they affect deer too. In one 2020 case in Pennsylvania, three captive white-tailed deer in otherwise good physical condition were found dead from heavy winter tick burdens, and a fourth had to be euthanized due to extreme lethargy. Milder winters favor tick survival, so as winters warm in some regions, tick pressure on deer is likely increasing.
Roads, Vehicles, and Habitat Edges
Between 1.5 and 2.1 million deer-vehicle collisions occur every year in the United States. That figure makes roads one of the most consistent non-natural sources of deer mortality across the country, hitting deer of all ages year-round with peaks during the fall breeding season when deer move more recklessly.
Beyond direct collisions, the way roads and development fragment habitat creates subtler survival problems. Research in Delaware found that deer living closer to habitat edges faced significantly higher mortality risk, likely because edges concentrate both human activity and hunting pressure. Older deer in the study actively avoided habitat edges, suggesting they learn over time that these areas are dangerous. Deer on public land also faced higher mortality than those on private land, a pattern tied to differences in hunting access and intensity.
Hunting as Population Control
Regulated hunting is one of the largest sources of adult deer mortality in the United States. Minnesota alone recorded over 186,000 deer harvested in the 2025 season, and similar numbers play out across dozens of states each year. Nationwide, hunters take millions of deer annually. Wildlife agencies set harvest quotas specifically to manage population size, which means hunting functions as an intentional form of population control rather than an unregulated threat. Still, from the perspective of an individual deer’s survival, hunting is one of the most likely causes of death for any adult animal, especially bucks.
Pesticides in the Environment
A less obvious but increasingly studied threat comes from agricultural pesticides, particularly a class of insecticides called neonicotinoids. Research at South Dakota State University exposed captive white-tailed deer to a common neonicotinoid at concentrations found in the environment and documented a cascade of effects. As pesticide levels increased in fawn tissues, survival dropped. So did thyroid hormone levels, jawbone length, body weight, organ weights, and overall activity levels in both adults and fawns.
The thyroid connection is especially concerning. In Montana, the prevalence of underbite (a shortened upper jaw) in white-tailed deer rose from 0% to 70% between 2000 and 2009, a deformity consistent with congenital thyroid disruption. Separately, 67% of male deer examined in one survey showed genital developmental abnormalities, including mispositioned or undersized reproductive organs. Researchers have pointed to endocrine-disrupting pesticides as a plausible cause, though proving a definitive link in wild populations is difficult. What the controlled experiments do show clearly is that field-relevant doses of these chemicals have direct, measurable effects on deer health and reproduction.
Perhaps most troubling, the South Dakota researchers found the pesticide present in organs of their untreated control group, meaning the deer were absorbing it from background environmental contamination before the experiment even began.