The nature of what black bears eat throughout the spring, summer and fall is varied, and reflects their opportunistic diet. Bears are omnivores, and take advantage of whatever is easily accessible and available to them. This, of course, changes with every season.
In general, less than ten percent of a bear’s diet consists of meat. Some years, a fawn may be taken in early spring, forest tent caterpillars may be eaten with relish and occasionally carrion is consumed, but most of a bear’s protein intake consists of the larvae and pupae of social insects, including ants, wasps and bees. The other ninety percent consists of the vegetation that is available at different times of the year.
Spring is very challenging for bears. When they emerge from their dens in April, they may have lost up to twenty percent or more of their weight over the winter. They are ravenous at a time when there is very little vegetation, the mainstay of their diet. Wetland plants such as swamp thistle are a primary source of food in early spring. Other foods include the tender, unfurling leaves of deciduous trees, catkins(pendant flowers) from early-blooming trees like aspen and willow, grasses, insect larvae and snowfleas. As time goes on, and pollinated flowers turn into edible fruits, the pickings improve.
Come summer, the nutritional value of vegetation begins to increase, and adult bears start to put on weight. Much of their diet consists of the ripened fruits of chokecherries, black cherries, blueberries, serviceberries, sarsparilla and spotted touch-me-not. Colonies of ants, wasps and bees are growing and a valuable source of food. As summer progresses, foraging takes on a whole new meaning. In the next two or three months bears must accumulate enough food to provide them with the four inches or so of fat that they need to sustain themselves for as long as seven-and-a-half months of not eating. By the time fall arrives, acorns and other nuts have matured, and provide bears with a high energy source of food. Acorns contain large amounts of protein, carbohydrates as well as fats, and are the nut of choice for black bears. Other nuts, including hickory and hazelnut, are widely consumed as well.
Bulking up to 300 or 400 pounds on morsels as tiny as ant larvae, snowfleas, raspberries and blueberries seems an indomitable task, but bears are up to it. At this time of year, bears eat continuously all day long -- they have been observed foraging for food for up to ten hours without stopping to rest. Bears go from consuming a normal 4 – 6,000 kilocalories a day to gorging down 18 -20,000 kilocalories a day (Terry DeBruyn, Walking with Bears) and gain as much as 30 pounds a week. This process of eating nonstop, referred to as hyperphagia, is essential to a bear’s winter survival. Starting in October (the greater the abundance of acorns and other nuts, the later bears retire to their dens), black bears will enter into true hibernation. While their temperature doesn’t drop drastically (100 F. down to 91 F., their heart rate ( 8 beats per minute during hibernation ) and their breathing (one breath every 45 seconds during hibernation) do. This metabolic slowdown is what allows them to go up to seven and a half months without eating – or drinking, urinating or defecating. Every acorn counts this time of year.