Until the next spring, when the new growth starts, they depend largely on the twigs and buds that are within their reach. Even in winter they consume green forage, for example, winter-green forbs, grasses, and sedges. Sometimes the move from summer to winter range requires travelling many kilometres. Even the most favourable winter concentration areas have a limited food supply. If there are too many deer using the area, the most nutritious fodder disappears rapidly, leaving foods of only marginal value for the remainder of the winter.
Deep snow worsens the problem.
At snow depths greater than 40 cm, deer find it increasingly difficult to move about freely and tend to follow previously broken trails. The quantity and quality of food that can be reached from these trails further limit nutritional intake at the very time that intense cold and difficult travel tend to increase the deer's energy requirements. Some of this energy requirement is met by conversion of the surplus fat stored during the late summer and early autumn. Once this remaining source of energy has been depleted, the deer's chances of survival until spring are lessens. If forage remains scarce and the deer begin to breakdown muscle tissue for energy, then the chances of survival become extremely poor.