All bears are omnivores (they eat both meat and plant food just like humans) and eat a wide variety of foods including plants, berries, insects, fish, marmots and occasionally larger mammals such as deer, elk and mountain sheep.  Because bears are built for power and digging and not agility they are not very effective hunters so any larger prey is usually obtained in the form of the very young, very old, sick or already deceased (bears commonly overpower better hunters such as wolves and mountain lions and steal their food).  

        Although 90% of a bear's food comes from plants, bears have not evolved to be quite as efficient as true herbivores such as ungulates (hoofed animals such as deer, elk, sheep, etc) at digesting plant matter.  Ungulates actually have dual chamber stomachs allowing them to essentially double digest their food so that they are able to efficiently extract all of the nutrients from plants.  Bears however only have a single chamber stomach (as do humans) making them not as efficient as many true herbivores at extracting nutrients from plant food.  Because of their dual chamber stomach ungulates are able to survive on winter vegetation which has very little nutritional content.

        Bears on the other hand must instead eat plants with more nutritional content.  These reasons are why bears hibernate in the winter.  Bears don't hibernate because of the cold and snow and they can easily survive in cold harsh weather but instead must hibernate because they're not the best of hunters (prey is sparser and even more difficult to catch in winter) and they don't have the digestive efficiency to survive on low nutritional winter plants.  In the winter they would expend more energy trying to catch the occasional prey and digesting nutritionally void plant matter than they would gain from these food sources.  Nature and time have instead decided that it's best to gorge and store in the summer and sleep and conserve in the winter.  

        Typically younger vegetation is more nutritional so bears usually focus on new growth areas.  Summer comes first at the lower elevations and usually on the southern facing slopes (in mountainous areas southern facing areas receive more sunlight in the northern hemisphere).  Therefore in late spring to early summer bears usually focus on lower elevation vegetation because snow still rules at the higher elevations and the plants are still of the low quality winter variety at higher elevations.  As summer progresses bears migrate from lower elevations to higher elevations and from southern facing slopes to northern facing slopes constantly in search of young nutritional plants.

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