In ‘To Kill a Mockingbird' the author uses the mockingbird to communicate her theme of the defense of the innocent. "Shoot all the bluejays you want, if you can hit `em, but remember it's a sin to kill a mockingbird," Atticus tells the children. To put it in more general terms, Atticus tells Jem and Scout that we must follow a moral law that commands us to spare the innocent. This moral law has obligations. Atticus must defend Tom Robinson even though he knows he will fail because he must do the just and moral thing. "Tom Robinson's case is something that goes to the essence of a man's conscience—Scout, I couldn't go to church and worship God if I didn't try to help that man…..before I can live with other folks I've got to live with myself. The one thing that doesn't abide by majority rule is a person's conscience," says Atticus to Scout. We see through this quote that protecting the innocent goes before anything and stands at the very root of man's conscience. Furthermore, the children must respect harmless Boo Radley and not tease him even though he appears strange and reclusive. As a response to all the Boo Radley teasing, Atticus tells the children to let Boo manage his own business. At the end of the novel Scout says in reference to Boo Radley, "Well it'd be sort of like shootin' a mockingbird, wouldn't it?" She realizes the need to defend the privacy of the innocent and vulnerable.

In this novel Tom Robinson and Boo Radley represent the innocence of the mockingbird. Mr. Underwood, the town's editor, compares Tom to a mockingbird after Tom's death. "He likened Tom's death to the senseless slaughter of songbirds by hunters and children..." Tom's good character and total innocence make him resemble the harmless songbird. Not understanding that his best chance remains to bide his time and failing to grasp anything other than his crushing desire to escape, Tom seems like a frantic, caged songbird. Boo Radley, too, resembles a mockingbird because of his natural desire to please the children with the gifts he puts in the tree and his tender compassion for them when they stand outside shivering in the cold watching Miss Maudie's fire. As Miss Maudie says, "Mockingbirds don't do one thing but make music for us to enjoy." Furthermore, the night Boo Radley saves their lives Jem and Scout hear a mockingbird in the Radley's yard, "High above us in the darkness a solitary mocker poured out his repertoire in blissful unawareness of whose tree he sat in, plunging from the shrill kee, kee of the sunflower bird to the irascible qua-ack of a bluejay, to the sad lament of Poor Will, Poor Will, Poor Will." Both Boo Radley and Tom Robinson pour out their songs of joy or sympathy in a kind of blissful unawareness of the consequences.

Finally, we may even see Mayella as a fallen mockingbird, a mockingbird that has lost its innocent song. We see her attempts to bring something harmless and good into her world simply by planting bright geraniums and by trying to keep herself clean. She lost that innocence by essentially killing Tom Robinson in order to save her honor. Atticus seems to wish that he could spare Mayella on the witness stand, "Atticus had hit her hard in a way that was not clear to me, but it gave him no pleasure to do so." He says in his final remarks, "I have nothing but pity in my heart for the chief witness for the state." Potentially every character in this book could be a mockingbird. However, once racial prejudice clouds a person's mind it quickly becomes impossible for that person to resemble a true mockingbird. This appears the essence of why Atticus knows he must try to protect the innocent of whatever kind because if he does not protect the innocent, he may lose the spirit of the mockingbird that lives in him.