Seasonal range selection in bighorn sheep: conflicts between forage quality, forage quantity, and predator avoidance.

The migratory and foraging behavior of individually marked bighorn ewes (Ovis canadensis) was studied to test the hypothesis that forage quality determined seasonal range selection. Forage quality was monitored through analysis of fecal crude protein. Ewes in the study population utilized two distinct ranges differing in elevation and possibly predation risk. Pregnant ewes migrated in May from the low-elevation winter range to lambing areas at higher elevation, before plant growth had started there. In so doing, they moved from a range of high-quality forage to one of low-quality forage, apparently to avoid predation on newborn lambs. Non-pregnant adult ewes migrated later. Most yearling ewes (which are not pregnant) migrated with the adult ewes to the lambing areas, but returned to the winter range within a few days, then migrated again to high-elevation areas in June. Forage quality was higher at high elevation from mid-June at least through July, but forage availability appeared to be lower than in the winter range. Seasonal range selection is likely determined by a combination of nutritional and antipredator constraints. The antipredator strategy of bighorn ewes does not always allow them to utilize the range with the best forage.