How does the novel title now connect to both Tom Robinson and Boo Radley?

Tom Robinson and Boo Radley both serve as mockingbirds, innocents who do nothing but hope to please other people. It was a sin for Tom Robinson to be convicted of a crime he didn't commit, a conviction that ended with his death. It would have been a sin to drag Boo into an inquiry about Bob Ewell's death.... for he was an innocent protecting the innocent.