Is expiation obligatory upon one who, out of ignorance, caused the death of a pet rabbit due to cold weather?

There is no expiation (kaffara) or sin upon you for removing a pet from the house, for Islamic law does not oblige you to bring it in or prevent you from taking it out. Sin occurs when an animal is imprisoned and deprived of food and drink until it dies, as mentioned in the Hadith about a woman who was tormented because she imprisoned a cat until it died. Being kind to a pet brings reward and charity.