At the beginning of chapter 10, Jem and Scout are shooting their air rifles outside, and Atticus tells the children that he would prefer if they shot at tin cans in the back yard. Atticus then tells Jem and Scout,
"Shoot all the bluejays you want, if you can hit ‘em, but remember it’s a sin to kill a mockingbird." (Lee, 93)
Scout is surprised by Atticus's comment about shooting at mockingbirds and says it was the first time she heard him consider anything a sin. Miss Maudie elaborates on Atticus's statement by saying,
"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." (Lee, 93)
According to Atticus and Miss Maudie, it is considered a sin to kill a mockingbird because they are friendly, benevolent birds and cause nobody harm.
Throughout the novel, mockingbirds symbolize any innocent, compassionate being who is vulnerable and cannot defend themselves against others. Tom Robinson and Arthur "Boo" Radley are two prime examples of symbolic mockingbirds in the story. Both Tom and Boo are innocent, generous individuals who are defenseless against their community's prejudice. Tom Robinson relies on Atticus's help to defend him against a racist jury and Boo Radley relies on Sheriff Tate to protect him from the community's limelight. Essentially, Atticus's comment about it being a sin to kill a mockingbird is a metaphorical lesson on the importance of protecting innocent, vulnerable beings. By the end of the story, Scout metaphorically applies her father's lesson regarding Boo Radley's unique situation by telling him,
"Well, it’d be sort of like shootin‘ a mockingbird, wouldn’t it." (Lee, 280)