Harper Lee uses bird symbolism throughout To Kill a Mockingbird, establishing the mockingbird as a symbol of innocence after Jem and Scout receive air rifles for Christmas. Atticus tells Jem to shoot all the blue jays he wants,
"... but remember, it's a sin to kill a mockingbird."
Miss Maudie elaborates Atticus' warning when she explains to Scout that
"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."
When death is eminent for Tim Johnson, the mad dog who Atticus is forced to kill, the innocent "mockingbirds were silent," as if already in mourning for the loss of a fellow animal. As the jury prepares to give their verdict, Scout senses that the
... atmosphere in the courtroom was exactly the same as a cold February morning, when the mockingbirds were still...
No birds sing for Tom, the innocent victim of malicious charges by the Ewells. After Tom's death, B. B. Underwood's editorial likened Tom's killing to
... the senseless slaughter of songbirds by hunters and children.
Even Scout recognizes the validity of Atticus' warning after Sheriff Tate decides to call Bob Ewell's death self-inflicted. Charging Boo Radley--another of the novel's innocent human mockingbirds--with Bob's death would have been
"... like shootin' a mockingbird."
Lee established her bird symbolism with the main characters, the Finch family. The finch, like a mockingbird, is a songbird whose main purpose is to sing and make people happy. Most of the children in the novel are symboized as human mockingbirds--innocent kids who are forced to witness the evil around them; Tom and Boo are other human mockingbirds, both innocent men accused of crimes they did not commit.