To Kill a Mockingbird is the title of a book (and later movie) by Nelle Harper Lee. The title is referred to in the book in a discussion
about killing innocent creatures and that it is a "sin to kill a mockingbird" because it does nothing to harm anyone and, in fact, its purpose is only
to sing beautifully for us. The theme is part of the story. You can read it
for yourslef itis one of my favorite books.