There are many quotes that point to Boo as a symbol of a mockingbird. One example of his "mockingbird" status is the connection between him & the tree. He leaves gifts there for Jem and Scout- his version of the mockingbird's singing. When Jem sits down to write Boo the letter, he says "we appreciate everything which you have put into the tree for us." This shows that while they may not yet guess who's doing it, Jem & Scout know that someone innocent and kind is offering friendship.
This lesson is first revealed when Atticus buys the children guns.
Atticus said to Jem one day, “I’d rather you shot at tin cans in the back yard, but I know you’ll go after birds. Shoot all the bluejays you want, if you can hit ‘em, but remember it’s a sin to kill a mockingbird.”
That’s the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it.
“Your father is 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.”
Without realizing, Jem, Scout, and Dill have in a way been trying to kill that mockingbird. The children have built up a view of Boo based solely on a preconceived notion that is completely wrong. This is similar to the racism evident in the rest of the town: a judgment made before knowledge. Yet from that notion they have developed a pattern of behavior that seeks to destroy the true nature of Boo by pretending that it could not possibly exist.
It is through the attack on the children and Sheriff Tate's understanding that Boo’s identification as a mockingbird becomes most clear. The sheltered innocence of Boo’s life would be threatened should he be brought to trial for the death of Bob Ewell, even though he would most likely be acquitted as a hero. It is this hero worship that would “kill” the mockingbird, Tate believes. The people, especially the women, would bother him continually with food and praise for such a brave act. Such attention would ultimately destroy who Boo is, his innocence, and his quiet love for the children of Atticus Finch. Scout and Jem finally acknowledge this at the end. When Atticus asks Scout if she understands why they will say Bob Ewell fell on his own knife, she replies that is they did it any other way, "Well, it'd be sort of like shootin' a mockingbird, wouldn't it?
Tom's connection is not as obvious; it is inferred from his character, rather than directly stated. Innocent of the crime for which he is tried, he valiantly attempts to prove that innocence to the society which has already judged him. His noble life has brought joy to those with whom he has come into contact, but for most in the town, his skin color has already convicted him. Even with Mayella Ewell, he has tried to help her, something which her family cannot seem to do. Yet for his attempt he is killed, crushed by the racism that is so deeply embedded in the community. With his death, Dill, Jem, and Scout come face to face with the sin that has killed this mockingbird.