A "mockingbird" in this book symbolizes a couple things.
First of all, it's called 'To Kill a Mockingbird' because Atticus told Jem "'Tis a sin to kill a mockingbird" because all they do is sing and don't harm anyone.
First of all, Jem is a mockingbird because at the beginning of the book, he's so innocent and naive. However, after the trial, his innocence is shattered and destroyed because he believed that Tom Robinson would win, but he didn't because he's a N e g r o. The trial made Jem more aware of how cruel the real world can be.
The second "mockingbird" is Boo Radley because he's a scapegoat of Maycomb County. In chapter one, there's a physical description of Boo Radley, saying that he's 6.5 feet tall, blood on his hands, scars, etc. Towards the end of the book, we learn that these attributes/characteristics that Boo is said to possess are incorrect. Boo turns out to be a completely different person than the Boo Maycomb County has created.
The third "mockingbird" is Tom Robinson. He was blamed for the rape and abuse of Mayella Ewell only because he's black. He's another scapegoat of Maycomb County, just because of his race.
And lastly, I would say Dolphus Raymond is another example of a "mockingbird" because he married a ***** woman and has mulatto children. He is ostracized by Maycomb County because they blame his marriage to a ***** woman on his "whisky", which is in fact Coca-Cola. He's excluded from both the white and N e g r o community, because he cannot fit into either groups.
I hope this helps, and if you have any other questions, feel free to ask :)
PS - The words with a bunch of *'s are N...E...G...R...0
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