Deer are commonly believed to be strict herbivores, peacefully grazing on plants and leaves throughout their lives. While this perception is largely accurate, it overlooks a surprising, scientifically documented aspect of their behavior. Deer do occasionally consume birds and other animal products. This is not a standard part of their diet, but rather a rare, opportunistic behavior.
The Primary Diet of Deer
Deer are classified as ruminants, possessing a complex, four-chambered stomach designed to process large quantities of plant material. The rumen, the largest chamber, functions as a fermentation vat where specialized microbes break down cellulose into usable nutrients and energy sources. This system allows them to rapidly consume forage and later retreat to ruminate for complete digestion.
Deer are also known as “concentrate selectors,” meaning they choose the most nutritious and easily digestible plant parts. Their typical diet consists primarily of forbs, tender shoots, leaves, buds, and woody browse high in protein, fat, and starch.
Defining Opportunistic Carnivory
Despite being adapted for a plant-based diet, deer exhibit a behavior known as opportunistic carnivory. This term describes the occasional consumption of animal matter encountered by chance, rather than through active hunting. The behavior is rare and context-dependent, but it is scientifically validated across various deer species.
This dietary flexibility includes consuming eggs and nestlings, or scavenging on dead animals like fish or small mammals. Deer lack the sharp teeth or digestive enzymes required for a regular meat diet, making them different from true carnivores. Their consumption of animal products represents nutritional scavenging, taking advantage of an easily accessible meal that provides dense nutrients.
The Nutritional Drivers
The primary motivation for this unusual feeding behavior is typically a deficiency in essential minerals often scarce in plant matter. The most common drivers are the need for calcium, phosphorus, and sodium, which are vital for a deer’s health. These minerals are not always adequately supplied by forages, especially in mineral-poor soils or during certain times of the year.
Calcium and phosphorus are necessary for bone growth, milk production in females, and the rapid growth of antlers in males. Antlers create a high demand for these nutrients during the spring and summer growth phases. Pregnant and lactating does also require a high intake to support gestation and milk production. Animal remains, eggs, and bones provide a concentrated, bioavailable source of these elements.
Documented Examples of Non-Vegetative Consumption
The instances of deer consuming non-vegetative material are well-documented, often captured by trail cameras or observed directly by researchers. One specific behavior is osteophagy, which involves gnawing on bones or shed antlers to extract calcium and phosphorus. This practice highlights their targeted pursuit of dense mineral sources outside of their typical plant diet.
Direct evidence of consuming birds specifically comes from nest predation studies. White-tailed deer have been caught on camera consuming eggs and chicks from ground nests, sometimes raiding more nests than traditional small predators. Observations have included white-tailed deer foraging on Northern Bobwhite eggs and red deer biting the heads off Manx shearwater chicks to consume the high-protein, mineral-rich bone tissue.