My chicks have been outside in their new enclosure for about 5 days now, but every night they don't go in the coop to sleep. they just huddle on the ground. I've been moving them up to the the coop every night where they have a heating pad, but they haven't got the hint yet to do it on their own. They can go up and down the ramp just fine as they do it all day long. Is this behavior strange? Any hints on how to get them to go up there on their own?