Deer's stomachs and metabolism change to prepare them for winter survival. The food they eat in the winter is different than that which they find in the summer. They eat high protein foods during the warm weather and low protein foods in the winter. According to stuff I have read they have a difficult time digesting food like grain and other high protein food in the winter and can die of starvation with full bellies of the wrong type food. If you want to do anything for them especially when the snow gets deep cut some cedar boughs down so they can get to it. They often winter in cedar swamps due to the shelter it provides and the food available. Supplemental feeding creates an unhealthy diet and tends to group them together often promoting disease and over browsing conditions. Do them a favor and leave them alone. They will be better off in the end.