The title is an allusion to something that Atticus tells the children. He says that it is a sin to kill a mockingbird because mockingbirds do nothing but sing. They don't harm anyone or anything. What he means is that it is a sin to hurt someone who is innocent and has done nothing wrong. Several characters could be considered as "mockingbirds" in the novel. First, Atticus Finch does his best to provide a defense for Tom Robinson; however, many folks in town disagree with his decision to do his utmost for a Black man. Atticus holds his head high because, like the mockingbird, he is doing what he does best and is causing no harm to anyone. Tom Robinson could also be connected to the mockingbird because he did nothing wrong to Mayella Ewell, yet he was charged with raping and beating her. His death is a sin that the town of Maycomb will have to carry. Also, Boo Radley might be considered a mockingbird because he does nothing but stays in his house and harms no one (although rumors state otherwise). He is talked about in hushed tones as if he were some kind of freak.. not a kind way to treat a shy recluse. All three were innocent of doing any harm to anyone, yet they were treated badly by others.