EFFECTS OF SNOW ON FORAGING ECOLOGY AND NUTRITION OF BIGHORN SHEEP

We studied foraging behavior, diet quality, and activity budgets of Rocky Mountain bighorn sheep (Ovis canadensis canadensis) under different snow cover conditions on a Ponderosa pine (Pinus ponderosa) -shrub-bunchgrass range in the montane zone of Colorado. Snow 2-12 cm deep persisted for less than a day to several weeks and caused bighorn to nose or paw to reach forage. Biting rates were reduced during periods of snow cover as were dry matter intake rates, despite an increase in bite size. Percentage of green material in the diet declined as snow depth increased, as did percent protein and dry matter digestibility of diets. During periods of snow cover, bighorn shifted from feeding in open sites where green-up was advanced to feeding in areas of shrub cover, which enhanced forage accessibility. Sheep partially compensated for declines in nutrient intake rates by increasing foraging time during the day. We concluded that even small amounts of snow had an important negative effect on foraging efficiency and diet quality of bighorn and caused major changes in their foraging tactics.