The title of the novel To Kill a Mockingbird refers to a saying that Atticus Finch passes on to his children, Scout and Jem. In chapter 10, after the children witness him kill a rabid dog with one shot, they discuss the ethics of shooting animals. He advises them that it would be acceptable to shoot a blue jay, but “it’s a sin to kill a mockingbird.” As Atticus otherwise does not speak about sin, Scout is confused. She consults their neighbor Miss Maudie. This kindly older woman confirms Atticus’s advice:
“Your father’s right,” she said. “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.”
The idea that killing any creature, human or otherwise, if that being only brings enjoyment is a key theme of the novel.
Two of the characters to whom this advice applies are Arthur “Boo” Radley and Tom Robinson. Atticus is concerned that his children are harassing Boo because their overactive imaginations consider him some kind of monster. Atticus emphasizes his humanity and encourages the children to have compassion. Boo is ultimately shown to be a lookout and protector of the children. Scout demonstrates her understanding of the mockingbird metaphor (chapter 30). Given Boo’s intense introversion, she offers that publicizing his actions would be like killing a mockingbird.
In regard to Tom Robinson, a “songbird” comparison is used by Braxton Underwood in a newspaper editorial after Tom is killed. This usage indicates that Underwood believed in Tom’s innocence, despite his conviction.