What governs the bison herd's decision regarding continuing to forage in snowy Yellowstone terrain?

Weighing the caloric return of the exposed grass against the energy spent using the head as a plow

Bison survival in winter is a complex, energy-conscious calculation, especially when heavy snow necessitates plowing. The herd does not continuously expend energy digging unless the reward justifies the cost. Foraging behavior is dictated by a constant evaluation process where the required expenditure of precious winter energy is rigorously weighed against the potential nutritional benefit gained. If the energy required to swing the massive head and plow through a specific snow depth or icy crust exceeds the caloric value that can be extracted from the uncovered cured grass, the effort becomes counterproductive. When this cost-benefit analysis tips too far toward expenditure, the herd will cease plowing at that location and actively move to find areas where the snow depth is more manageable or where natural shelter can reduce overall energy drain.