I'm thinking possum since the birds were nesting on the ground. They are scavengers. Birds have to be easy prey for a possum to get them. Do you know how they got inside?
Agree, sounds like a possum and you have a hole somewhere, so need to figure out where it is. I've had raccoons tunnel up into a building before that a groundhog had made. I don't know that a possum will go through that much trouble. Something could have killed the hen and the possum found...