Grizzly and Black Bear Diet Throughout the Season
Since bears spend much of their life sleeping away winter in their dens, they have only one main focus when they are active – building up sufficient fat layers for next winters siesta. Unlike ground squirrels, which are true hibernators, bears merely go into a heavy sleep in the winter. This means that their metabolism, heart rate and respiration drop only slightly below what would be normal for such large animals. The end result is that they require almost as much food energy each day they spend in their den as if they were actively moving around. Surprisingly, many bears continue to lose weight throughout the summer, even though they are up and actively feeding. In order to build up the fat layers required to sustain them through the winter, bears need foods with two characteristics. They must be highly nutritious and easily digested. During the annual cycle of life in the mountains, bears move with the availability of their primary foods.
Also, bears are not well adapted to eating plants. They are essentially meat eaters that have adapted to include a wide variety of plants in their diet. They do not have the ability, like the hoofed mammals, to digest cellulose, and so can only make use of the highest quality and most easily digested plant foods.
When grizzlies wake in the spring, they head to the avalanche slopes, make their way to the frozen food section and get themselves a TV Dinner. Over the course of the winter, the odd bighorn sheep, the odd mountain goat, and even the odd mountain climber falls victim to winter avalanches. These nicely preserved morsels give them the protein boost they need after their winter sleep. South-facing avalanche slopes are also one of the first areas to green-up in the spring, and so are attractive to bears.
Spring foods for black and grizzly bears also include last years berries (e.g. bearberry), green vegetation, flowers, insects, animals, and garbage. Oh yes, and don't forget to take down your bird feeder in the spring! Their diet is primarily composed of plants, and they actively hunt for the succulent new spring growth. As plants like the glacier lily, sedges, horsetails and many members of the Pea Family sprout, the bear will quickly find them. The early stages of these plants are more nutritious, higher in protein, and more easily digested than the latter stages after they flower, and so bears will actively seek out younger groupings of plants.
Surprisingly, at high elevation, plants grow much faster and this provides a situation in which there is a greater amount of protein in the plants when compared to lower elevations. As a result, both black bears and grizzlies prefer to feet higher up the slopes. Black bears climb tall poplars to eat the new buds. These tend to sprout near the tops of trees earlier than at the base.
Whitebark Pine seeds (pine nuts) can form another important spring food. Black and grizzly bears desire these seeds as they are exceedingly nutritious, high in protein and easily digested. While the bears cannot harvest the cones themselves, the cones are collected by squirrels and buried in their middens. Grizzlies can find and excavate these cone stashes through 2 metre snowpacks. In years following bumper cone crops, whitebark pine nuts can form a large part of the bears diet throughout the season.
In spring, when the high country is still snowbound, bears are forced into the valleys which contain both roads and man. As a result, we tend to see more bears, particularly grizzly bears, during the spring months before they retreat uphill with the receding snowline. An early drive up the Icefields Parkway at this time is often rewarded with the sighting of at least one bear – perhaps even a grizzly.
As the season continues, the south-facing avalanche slopes and meadows begin to dry out, and their value as a feeding location diminishes. The bears look for moist areas where plants may still be in their early development stages. Moving from south to north facing slopes can delay green-up by a month or two, so the bears follow the green-up with an uncanny sense of where the most food will be located. During this time, there is a greater potential for conflict with hikers and other outdoors people. During this brief period, the berries have not ripened, and their early season foods have disappeared. On the other hand, these mid-season locations represent some of our most popular alpine meadows. Deep, north facing, avalanche slopes will have a very late melt and a subsequently late green-up. Of particular value, the Cow Parsnip (Heracleum lanatum - photo at left) provides nutritious stems and occasionally leaf-stalks and flowers (rarely leaves). At lower elevations, black bears may feast on dandelions. Occasionally, roots of plants like the Hedysarum are critical to grizzlies. Black bears lack the long claws to excavate these underground roots and tubers.
Insects add to the diet. Where ever, how ever they can get them. Ants, beetles, bees, and anything else with six, eight or more legs. Since insects are prevalent in recent fire sites, caution is advised in these areas. In some areas, grizzlies will feed on large numbers of army cutworm moths by turning over stones to find the moths hiding during daylight hours.
Mammals, like ground squirrels and taken when available. Bears eat more than 80% plant matter, and animals are an opportunistic event. As the calving season begins for ungulates, black and grizzly bears are quick to arrive on the scene. In some areas, grizzlies can take up to 50% of newborn elk and 42% of moose calves. The calves quickly mature though and can elude bears by a few days of age. For the rest of the season, only visibly sick or injured animals stimulate the predatory attention of bears. Animals that have died by other means, either predatory or otherwise, are also attractive. Carrion forms a critical part of the bear's diet.
What about fish? Most people assume that grizzlies feed on salmon. Unfortunately for bears on the eastern slopes of the Rockies, there ain't no salmon here. They must rely on other sources of food. Farther west, salmon form an important food source during the annual run. In a few locations with narrow, shallow streams, trout may occasionally be taken.
On the western side of the continental divide, there is a diversity of berries that ripen and offer their succulent fruits to bears. These include currents, gooseberries, cranberries, huckleberries, bilberries, blueberries, salmonberries and even blackberries.
On the eastern slopes where there is a much more limited berry selection, berries like buffaloberry (Shepherdia canadensis) begin to ripen in mid-August. This is the peak feeding season. Finally we have plentiful, nutritional foods. Since buffaloberries have only a single seed, biologists can count the seeds in scats and get a great idea of the number of berries ingested (along with estimates of the number of daily defecations), biologists can estimate the number of berries and--and the numbers are immense. Bears will eat for up to 16 hour/day during this peak season and finally their weight begins to rise. They may feed on 50,000-200,000 buffaloberries each day. One study recorded an increase of 0.6 kg (1.4 lbs) per day during a 16 day period. In places like Banff National Park, where buffaloberry is plentiful, researchers have reported as many as 9 grizzlies within 3.1 sq. km (1.2 square miles). Huckleberries are also popular, along with sarsaparilla, Devil's club, raspberry, strawberry, and crowberries.
As berries begin to fall from the branch with the approach of cold weather, grizzly bears begin to dig again for Hedysarum roots and related tubers until they head into their dens for the winter. Whitebark pine nuts also form an important autumn food supply as grizzlies raid cone stores created by squirrels. Also during autumn, late ripening berries like those of the crowberry and low-bush cranberry rise in importance. In years of berry crop failures, they may feed on high numbers of sweet clover plants. Black bears do not have the ability to excavate beneath the surface like grizzlies, and so cannot dig for Hedysarum roots, but must instead rely on other food sources such as pine nuts, berries and sweet clover.
All Material © Ward Cameron 2005