Why does Atticus Finch say it is a sin to kill a mockingbird?

In Harper Lee’s novel To Kill a Mockingbird, Atticus Finch tells his children that it is a sin to kill a mockingbird because these birds are innocent and harmless creatures that do nothing but provide music for people to enjoy.12

This statement serves as a central moral metaphor throughout the story. By extension, the "mockingbird" represents vulnerable individuals who are innocent and do not cause harm to others. The novel uses this symbolism to highlight the tragedy of those who are unfairly persecuted or destroyed by the prejudices of society. For example, the death of Tom Robinson is later compared to the "senseless slaughter of songbirds," and the character Scout eventually recognizes that exposing the reclusive Boo Radley to public scrutiny would be "sort of like shootin’ a mockingbird".3