Bingo.

Their are alot of species of berries that whitetail do not normally eat. But during winter months with less food available deer are apt to eat many berry species including the buckthorne. Many of the berry species are not eaten during times where alternative food is plentiful because these berry species cause irregular bowl movements and/or diarhea in deer.

Now.. whitetail will still forage on the Buckthorn and some of the other species of berry even in times of plentiful alternative food sources (not nearly as often). But without the snow.. simply put.. its ALOT harder to identify their pee color.