In Chapter 10 of Harper Lee's To Kill a Mockingbird, Scout characterizes her father as not doing anything:
He worked in an office, not in a drugstore. Atticus did not drive a dump-truck for the county, he was not the sheriff, he did not farm, ...or do anything that could possibly arouse the admiration of anyone.
Indeed, Atticus is not ordinary, not like other residents of Maycomb. When he does the usual thing of purchasing air-rifles for his children, he yet neglects to teach them to shoot. It is their Uncle Jack who instructs them, explaining that his brother is not interested in guns. Atticus does tell the children that he would rather that they shoot tin cans in the back yard. They can also shoot all the bluejays that they wish, but they must not kill a mockingbird because it is a sin to do so; later, Miss Maudie explains to the children,
"Mockingbirds don't do one thing but make music for us to enjoy...They don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird."
Here in this chapter the mockingbird is introduced as a symbol of purity of nature, of which both Boo Radley and Tom Robinson are possessive. For, neither of them intend any harm to others.