I just recently started keeping mice a few months ago. I give mine nesting boxes, such as cereal, kleenex, soup, etc boxes. They keep the babies in there. I remove the bedding on the other side and they will kick the old bedding out of the box and replace it with the new stuff. Every other day or so I'll put a new box in the tank. They kick the old box away, move the new box into their space and fill it with clean bedding and place the babies inside. It's really interesting to watch them. They're extremely intelligent little creatures.