Mama should basically handle everything, as far as food and water, but you may want to keep her and the chicks in a small area, so the chicks can get under her easier if they get cold. That's why I took Gertie's last chicks away. The only 'pen' I had to put her in was a large dog kennel, and I couldn't see locking her up in there. Every time I saw those poor babies they were fluffed up. Don't let her go outside with them at all. They would get wet and chilled by the snow.
As for how they hatch? I have no idea, but they seem to manage. I think the hen kind of 'stands up' a bit to give them room.
As for how they hatch? I have no idea, but they seem to manage. I think the hen kind of 'stands up' a bit to give them room.