I got my first chickens about a week ago-have never had chickens before. The flock is 8 older birds that someone gave me-6 hens, 2 roosters-these birds have been together for about 2 years. I kept them in their coop and pen for a week so that would learn they are home. I let them out of this afternoon into the yard so they could roam a little. I had planned on letting them have the run of the yard each day. They wouldn't leave their pen--is this normal? How can I encourage them to leave the pen?