Author Harper Lee uses the mockingbird primarily as a metaphor for innocence in To Kill a Mockingbird. As Miss Maudie explains,
"Mockingbirds don't do one thing but make music for us to enjoy... That's why it's a sin to kill a mockingbird."
Tom and Boo are both symbols of the mockingbird. The editor of the newspaper, B. B. Underwood, compared Tom's death to the "senseless slaughter of songbirds." Late in the story, Scout recognizes how Boo Radley is also like a mockingbird when she agrees with Sheriff Tate's decision to call Bob Ewell's death self-inflicted instead of publicizing that it was Boo who had killed Ewell in self-defense.
"... it'd be sort of like shootin' a mockingbird, wouldn't it?"
Boo and Tom both represent innocent men who have been attacked by others--both metaphorically and physically. Scout, Jem and Dill also represent the innocence of the mockingbird; as innocent young children, their primary purpose is to bring joy to their parents and the world around them, yet, they, too, face the wrath of others who would harm the harmless.