Bighorn sheep diet selection and forage quality in central Idaho.

Rocky mountain bighorn sheep (Ovis canadensis canadensis) populations in the Big Creek drainage of central Idaho spend the winter on low elevation, southern exposures where snow depths are reduced or absent. Approximately 50% of the population migrates to higher elevations during warmer months, thus potentially extending the period of spring green-up and its accompanying higher levels of forage nutrient content and digestibility. Other bighorns remain on low elevation winter range year-round, experiencing warm, dry conditions and lower forage nutrient values earlier than sheep migrating to higher elevations. The objective of this study was to determine and interpret the seasonal diet selection of the non-migratory portion of the population. Microhistological analysis of composite fecal samples was used to determine diet composition, and plant samples were analyzed for crude protein, digestibility, and macro and micro nutrients to examine forage quality. Graminoids made up the majority of the diet throughout the year. During spring green-up when protein content and digestibility of grasses were at peak levels, consumption of forbs and browse declined. However, forbs and browse provided important sources of nutrients, especially protein, at critical times of the year when grasses were low in nutritional value and digestibility. Non-migratory bighorns had developed flexible and dynamic feeding behaviors that allowed them to meet their nutritional needs while remaining in a relatively warm, dry environment. Managers should focus on providing a diversity of plant species in all forage categories. Invasions of exotic plant species that reduce biodiversity may negatively impact bighorn sheep populations. Author to whom correspondence should addressed. E-mail: gwagner@cdatribe-nsn.gov Current Address: Coeur d’Alene Tribe Wildlife Program, 850 A Street, P.O. Box 408, Plummer, ID 8385