Mockingbirds are a symbol in this novel, used to represent qualities of harmlessness and a need for protection. On several occasions, these qualities are discussed and the mockingbird is defined as a victimized figure, persecuted for no good reason. 

Tom Robinson and Boo Radley are the two characters most closely aligned with the mockingbird. They offer aid in the book and never any harm. Yet, they find themselves in need of protection. In this way, they fit the mold of the symbolic mockingbird.

As Miss Maudie Atkinson explains, it would be thoughtlessly cruel to kill innocent creatures that "don't do one thing but make music for us to enjoy."

Tied to a larger idea relating to justice, the mockingbird becomes part of a conversation on morality. For justice to be done, good people have to act. The mockingbird must be protected by those who realize it is in need of protection. Standing back and hoping that justice will be done is not enough. 

Atticus Finch is the character that presents the clearest example of this ethic.

Atticus has a highly develop morality, with very strong convictions about wrong and right. He is prepared to stand up for what is right, regardless of the consequences.