Is there a sin upon one who was negligent in feeding an injured pigeon and forgot about it, and what is the expiation for that?

If the pigeon is caged, its keeper must feed it, and he is sinful if he leaves it until it dies. He must seek forgiveness, and there is no expiation. However, if it is not caged, then feeding it is not obligatory, and there is no sin upon whoever leaves it, especially if he has provided food for it.