The first reference to mockingbirds in the book occurs when Jem and Scout get air rifles from Uncle Jack for Christmas in chapter 10. Atticus is not too thrilled by this gift but understands that it is a common toy for children. He instructs his children that they should never use their new air rifles to kill a mockingbird because it would be a sin to do so. Scout is struck by her father's instructions because he never calls anything else sinful. Miss Maudie explains just what Atticus means when she tells Scout,
Mockingbirds don't do one thing but make music for us to enjoy … That's why it's a sin to kill a mockingbird.
Throughout the book, two people are overtly and subtly compared to mockingbirds. They are nice to others but are commonly misunderstood and ill-treated. The first is Tom Robinson. He is a kind man who goes out of his way to treat everyone well, even those who wish him harm. Despite this, he is falsely accused of rape, convicted in court, and killed. The other mockingbird is Arthur (Boo) Radley. This reclusive neighbor of the Finch family seems to desire nothing but to be left alone. He does, however, surreptitiously do nice things for the kids, such as mend Jem's torn pants and leave them gum in the tree hole. After Boo kills Bob Ewell to save Jem and Scout, Atticus and Sheriff Tate decide to hide the truth to protect Boo from unwanted attention. With this, Scout makes a direct connection to what her father said about mockingbirds:
Mr. Tate was right … It would be sort of like shooting a mockingbird, wouldn't it?