I have had our chicks now 3 weeks old in the coop for a couple weeks but just since Saturday been letting them out in the attached secured run. When I go out there near dinner time, when it is starting to get cooler (worried about them being cold) I find that some of them are huddled under the coop but the rest went back in. Should I be worried? I have been picking them up and putting them back in but wonder if they will do it on their own? Thanks.