We had a hen who did this also. From the time the kittens were born, she would sit on them and keep them warm while the mother was out hunting, and then they'd trade places while the hen ate so the cat could feed her kittens.
Mother cat with baby ducks and her kittens.What a beautiful photo! (KO) What a good little mama cat. Mothering what would normally be a prey animal. They are babies and obviously need a mama, so kitty steps in as substitute. Sweet babies all.