I suspect they want to be as close as possible to the 'bigs' - without getting too close to get pecked/thrown off. That is kind of sweet. If they had an actual momma hen, they probably wouldn't have ventured up at night so quickly because they would have tucked under her...and she would have been calling them to bed with her if the didn't come on their own. My chicks ( 3 weeks - so a week older than yours) will perch on the roosts at times during the day, but all hop into the nest box with mom at night.