I am new to raising chickens and will be getting 35 or so cornish crosses here pretty soon...I have my chicken tractor built and was planning on fencing off a pasture area, but I have had several people tell me recently that it wasn't necessary to fence them in. I was told that they just run free during the day time, and they automatically go back into the coop at night. Is this true, and what are the advantages/disadvantages to letting them roam free vs. fencing them in?