I'm totally new to raising Chickens. We started yesterday with 3 ISA Browns chicks. I purchased this coop today but it doesn't have a door that closes to keep the chickens in the coop. I see that a lot of coops do have doors that people must close. You can see where they walk up the ramp to get into the coop, but there isn't a door over that (approx.) 10" x 10" opening. What is the purpose of the door and is it needed or not? We live in Michigan where winter nights get to 10 degrees. Thank you for your help!