Some material from the National Park Service.
JACKSON HOLE, WYO – Bears are omnivores so by definition just about anything edible could be in their stomachs at any given time. But seasonal availability and certain favorite foods means a few grocery list items are high on the list of what a bear might be snacking on.
Overall, army cutworm moths, whitebark pine nuts, ungulates, and cutthroat trout are the highest quality food items available to grizzly bears in the Greater Yellowstone Ecosystem (GYE). These foods impart the greatest nutritive value in exchange for the least foraging effort.
Interestingly, studies have shown bears lack a cecum, meaning they do not digest vegetation very efficiently so bruins have learned to forage for plants that are in the phenological stages of highest nutrient availability and digestibility.
From March through May, ungulates, mostly elk and bison, comprise a substantial portion of a grizzly bear’s diet. Grizzly bears feed on ungulates primarily as winter-killed and wolf-killed carrion but also through predation on elk calves. Some larger male grizzly bears also prey on adult bison during early spring.
Grizzly bears also dig up pocket gopher caches in localized areas where they are abundant. Other items consumed during spring include succulent grasses and sedges during early green-up, dandelion, clover, spring-beauty, horsetail, and ants.
During spring, grizzly bears will also feed on whitebark pine seeds stored in red squirrel caches during years when there is an abundance of over-wintered seeds left over from the previous fall.
From June through August, grizzly bears continue to consume succulent grasses and sedges, dandelion, clover, spring-beauty, horsetail, and ants. In addition, thistle, biscuit root, fireweed, fern-leaved lovage, and army cutworm moths are eaten. Predation on elk calves continues through mid-July when most grizzly bears are no longer able to catch calves. In areas surrounding Yellowstone Lake, bears feed on spawning cutthroat trout.
Starting around midsummer, grizzly bears begin feeding on strawberry, globe huckleberry, grouse whortleberry, and buffaloberry. By late summer, false truffles, bistort, and yampa are included in the diet, and grasses, sedges, and dandelion become less prominent.
Throughout the summer, grizzly bears scavenge the remains of wolf-killed ungulate carcasses usurped from wolf packs. In late summer during the breeding season, grizzly bears scavenge the carcasses of bull bison that have been gored and die while competing for female bison.
From September through October, whitebark pine nuts are the most important bear food during years when seeds are abundant. However, whitebark pine is a masting species that does not produce abundant seed crops every year. Other items consumed during fall include: pond weed root, sweet cicely root, bistort root, yampa root, strawberry, globe huckleberry, grouse whortleberry, buffaloberry, clover, horsetail, dandelion, ants, false truffles, and army cutworm moths. Some grizzly bears prey on adult bull elk during the fall elk rut.