Bears are highly opportunistic omnivores, and avian life—including eggs, nestlings, and sometimes adult birds—can become part of their varied diet. This consumption is generally secondary to their main food sources, but it is a documented part of the foraging behavior for several bear species. The extent of this feeding depends greatly on the bear species, the season, and the local availability of other, more calorie-dense food items. Bears constantly seek the easiest and most substantial nutritional payoff in their environment.
The Primary Target Eggs and Nestlings
Eggs and young birds represent a highly efficient food source for a foraging bear, offering a rich concentration of easily digestible nutrients. Bird eggs provide a complete protein profile, healthy fats, and a dense array of vitamins and minerals. This high caloric and protein density makes them a premium target during the bears’ active feeding seasons.
Finding a nest full of eggs or flightless nestlings is much easier and safer than actively hunting an adult, flying bird. Bears are not built for agile pursuit, lacking the speed and coordination needed to consistently catch adult prey. The stationary nature of a nest allows bears to secure a substantial meal with minimal energy expenditure or risk of injury. Ground-nesting birds, such as waterfowl and galliforms, are particularly vulnerable because their nests are easily accessible to a terrestrial predator.
Predation Patterns in Black and Brown Bears
American Black Bears (Ursus americanus) and Brown Bears (Ursus arctos), which include the Grizzly Bear, both exhibit opportunistic avian predation, but their methods differ due to their physical attributes. Black bears are adept climbers, allowing them to access nests located higher up in trees, though they still most often target ground-level nests. Their predation is often a supplement to a diet heavily focused on vegetation and insects, such as ant larvae and hornet grubs.
Brown bears, being larger and less skilled at climbing, primarily focus on raiding ground nests. They enter marshy areas or coastal regions to consume the eggs and young of waterfowl like geese and sea ducks. For both species, the consumption of avian prey and eggs is typically a seasonal event, occurring most often in the spring and early summer when birds are nesting and other preferred foods, such as berries or spawning fish, may not yet be widely available. Subadult and female brown bears are sometimes implicated in destroying a high percentage of nests in dense nesting areas.
Avian Prey and the Polar Bear Diet
The Polar Bear (Ursus maritimus) presents a unique case, as it is the most carnivorous of the bear species, with a diet historically dominated by seals hunted on sea ice. The shrinking duration of sea ice in the Arctic is forcing polar bears to spend more time on land during the summer, leading to a distinct shift in their foraging behavior. During this ice-free period, they must seek out terrestrial resources, and migratory bird colonies provide a temporary but substantial food source.
Polar bears have been observed aggressively targeting large colonies of ground-nesting birds, such as snow geese and common eiders, and their eggs. A single polar bear can consume hundreds of seabird eggs in one sitting, having the capacity to devastate virtually all the eggs in a concentrated nesting area within a 48-hour period. This opportunistic egg consumption is an environmentally driven pattern, becoming more frequent as the bears’ primary hunting grounds on the sea ice disappear earlier in the year. Despite the large quantity of eggs consumed, this land-based diet is generally insufficient to maintain the high body condition polar bears achieve from high-fat seals.