What animal does Atticus say it is a sin to kill?

Atticus says it is a sin to kill a mockingbird. He gives this lesson to Scout and Jem after allowing them to receive air rifles, warning them that shooting harmless songbirds is morally wrong.

Miss Maudie explains the reason: mockingbirds do not destroy gardens, damage crops, or harm other creatures. They simply sing for people. In the novel, the bird therefore becomes a symbol of innocence and goodness that should be protected rather than punished.

The mockingbird image connects several characters and events. Tom Robinson is described as a mockingbird because he is harmless yet destroyed by prejudice, while Boo Radley is also associated with innocence misunderstood by the community. A common mix-up is the blue jay: Atticus says the children may shoot bluejays, but specifically forbids killing mockingbirds.