The first time a mockingbird is mentioned is in Chapter Ten, when Atticus speaks to Scout and Jem about their new rifles. Jem relates the following:
"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 was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it.
"Your father's 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."
In this context the symboloc meaning is obvious. Mockingbirds bring us pleasure by singing beautifully. They copy the sounds other birds make to add to our pleasure. The birds are deemed harmless and innocent and only wish to bring us joy. In this sense, then, the meaning aptly describes Boo Radley
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