Atticus tells Jem and Scout not to shoot any mockingbirds with their air rifles. Why?

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."