Winter
Winter in northern regions usually brings deep snow that limits the availability of most foods to deer. As a result, the winter diet of northern deer consists almost entirely (over 80%) of coniferous and deciduous browse and bark. Dry leaves, evergreen ferns, and plants with persistent winter fruits (e.g., mountain ash, hawthorn, rose hips, sumac, and poison ivy) are also important. Following an abundant mast year, deer may paw through more than one foot of snow to find and eat acorns. Once deep snow and cold temperatures are common, any plant that provides food within or near winter cover is typically consumed first, regardless of its palatability.
In winter, northern deer experience what is called a "negative energy balance" as a result of the poor availability and quality of winter food. This means that food quality is so poor that deer are unable to eat enough food to maintain their body weight, and as a result, they loose weight (mostly fat) throughout winter.


(Research has shown that even when northern deer are provided high-quality supplemental food during winter they still loose weight.)

http://www.whitetailstewards.com/articlesonsite/deerbiology/seasonalhabitatrequirementsnortherndeer.htm







Because winter food is of such poor quality, deer rely on stored body fat for as much as 30% of their winter energy requirements. Additionally, deer reduce the amount of food they require by reducing their activity and by selecting winter cover that provides shallow snow depths and warmer temperatures.