Based on the synopsis, which theme of the story is most likely symbolized by the word mockingbird in the books title?

When Atticus buys his children air rifles for Christmas, he tells them
"Shoot all the blue jays you want, if you can hit 'em, but remember, its a sin to kill a mockingbird."
The quote from Atticus was considered the highlight of the book, and the title is after all, "To Kill a Mockingbird". It likens Scout's trials to the "sin" of killing a Mockingbird.
Hope this helps.