Hi. I am in Maryland and we are having zero degree weather with -20degree wind chills today. This is my first winter with chickens. I have 2 Barred Rocks and 1 Rhode Island Red. I know that are hearty winter birds, however when I opened the door to the coop today they willingly came out in to the cold. Should I have any concerns? Or will they go In the hen house if they get cold? They do not free range. I am concerned about frostbite on their combs an feet. I need a little reassurance!