I'm guessing the "shivering" is actually their heart it kind of feels like they are shivering when really little, but they have a really high heart rate so nothing to worry over. If they get cold they will let out loud single peeps or chirps a second or 2 apart and kind of look around or look at thier brooder. If they are playing they are fine. Also they might snuggle you to keep warm when you have them out which is fine just listen for that cheep that says "I'm cold put me back". I've never had full grown chickens but I've started alot of small bunches of chicks over the years and they are alot of fun to try to tame down. By 3 or 4 weeks you can tell which ones you've handled more. They will not resist being picked up at all and may even come to or jump in your hand, the rest will kind of run from your hand. If you don't want pets, lol, watch how tame they get I've had 1 or 2 that would jump out of the brooder and come looking for me at 6 or 7 weeks.