What is the ruling regarding someone who intentionally killed a bird during adolescence and regrets it, and is expiation (kaffarah) due upon him?

It is not permissible to kill birds intentionally without a legitimate Shar'i reason. Whoever does so, being an adult, is sinful, and the expiation for that sin is sincere repentance. However, if the killer is not an adult, there is no sin upon him, but he must guarantee the bird if it belongs to another person by paying its value to its owner or his heirs. If this is not possible, then its value should be given as charity on his behalf.