I have 10 chicks that are 3 weeks old. I have been reading about integrating them with the rest of my flock. They have been in the coop for the past 2 weeks. The see the big chickens and the big chickens see them. I was wondering when do you let the little chicks out in the run or free range with the older ones? I have 2x4 welded wire as the run fence. About how old do the chicks need to be to not fit into those 2x4 openings? Thanks!