In this chapter Atticus tells his children that “it's a sin to kill a mockingbird”. What reason does he give for saying this?

Atticus tells Scout and Jem they can shoot their air guns at tins cans and bluebirds, but that it is a sin to kill a mockingbird. Miss Maudie affirms this, saying to Scout, "Your father's right. Mockingbirds don't do one thing but make music for us to enjoy. They don't eat up people's gardens, don't nest in corncribs, they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird."

Yeah the answers in the movie. He says that they only sing for us to enjoy and that they are innocent.