In Harper Lee's To Kill a Mockingbird, the kids get air-rifles from Uncle Jack for Christmas. Uncle Jack gives them basic instruction about how to use the air-rifles, telling them that Atticus isn't much interested in guns. (This is an ironic statement based on the need of Atticus' excellent marksmanship when the threat of a rabid dog enters the neighborhood.)
Atticus gives the children instructions about what they can and cannot shoot.
I'd rather you shot tin cans in the backyard, 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.
(*Killing bluejays probably reflects the predatory nature of bluejays to attack and kill other birds.) It is actually Miss Maudie that explains Atticus' stand on mockingbirds.
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.
This reflects a central theme of the story: symbolically, Tom Robinson and Boo Radley are seen as mockingbirds in the story: neither one has ever caused harm to anyone, and yet they are both victimized. Tom is tried and convicted for a crime he did not commit, and Boo has been victimized by his family—first by his father and then his brother, so he is nothing but a shadow of the young man he once was, but almost a ghost.
The idea behind protecting mockingbirds is that no harm should come to those who cause no harm.