No extra heat needed. but when first hatched until at least 4-5 days old even 4-5 " maybe a bit much to maneuver when trying to get back to mama, I usually remove the nest box completely And just let them have the nice pine bedding on the floor because almost lost a chick once when it fell out of the nest box which was sitting on the floor and couldn't get back into get warm. and mom didn't leave the rest of her brood to help it,. this was just a simple box with a 2" lip to hold the bedding inside.
I have no problem just letting them settle on the floor if that works! I had thought I"d let them stay in the box thinking that it was a smaller place for mom to warm up. I think I could kind of block off an area with a couple straw bales - or even a box with an opening to get in and out but NO LIP just flat on the floor open. (Probably just turn it upside down with the open top on the floor so they're on the litter and an entrance cut all the way to flat.) That could give her a smaller place to have to keep warm. But if that's not necessary I guess I don't have to do it.
So another question - if they're on the floor like that, how long before mom is going to want to get up higher and roost? Will she stay on the floor with them for awhile and how long?