What does Atticus say about mockingbirds?
After the children get their air rifles for Christmas, Atticus tells them he would prefer they shoot at cans. He recognizes, however, that sooner or later they'll try to shoot birds. He tells them that as fat as he is concerned, they can shoot at all the bluejays they want but to leave mockingbirds alone. He says it is a sin to kill a mockingbird.
Later Miss Maudie explains to Scout that it is a sin because mockingbirds are harmless. They don't get into crops or destroy plants, they just sing.
"Your father's right," she said. "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 but one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird"
This intrigues Scout because she has never heard Atticus describe anything as a sin before. (Of course, for readers it also references the title of the book.)
Atticus views it a "sin to kill a mockingbird because they don't do one thing but sing their hearts out for us."
When Atticus give Scout and Jem their air-rifles, he told them that:
“I’d rather you shot at tin cans in the back yard, 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.”
which the bold words is also the same name as the title.
By this, the implied meaning is that it is wrong to attack innocents for no reason at all. At this point he is really referring directly to the real, actual, literal bird jay-birds that lives in the trees, whom are cruelly hunted down by poachers. By killing "mockingbirds", it is like destroying innocence.
It was also supported by Miss Maudie when Scout ask her. She replied, saying:
“Your father’s right,” she said. “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.”
supporting Atticus earlier comments about the mockingbirds to be true