Here when the deer yard up it is to find shelter from the storms as well as to create trail systems to move around in when looking for food. These trail systems allow them to go through snow better. As when the snow gets deep they have there own paths. also when the snow is crusty on top it cuts the deer's leggs up, and the paths helps them avoid alot of that. But with coyotes the crusty snow is a problem as the deer will break through and the coyotes run on top making it easier for the coyotes to catch the deer.