Atticus warns Jem after the children receive their air rifles as Christmas presents that it's okay to shoot tin cans and even bluejays, but that "it's a sin to kill a mockingbird." Scout doesn't understand her father's reasoning, but Miss Maudie explains it to her.
"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."
Mockingbirds serve as a symbol of innocence and beauty--often in contrast to the sometimes cruel world in which they live--and author Harper Lee uses the songbird to represent several of the characters in the novel. Boo Radley, Tom Robinson and Dolphus Raymond are all human mockingbirds--kind men who are innocent of the accusations made against them. The children--Jem, Scout and Dill--are also mockingbirds--youngsters who witness the less-than-perfect adult world around them (Jem and Scout's last name--Finch--is also a type of songbird), and who suffer a loss of innocence at a far more youthful age than should be expected.
The mockingbirds protest in silence when the rabid dog is killed and when the jury announces its unjust verdict. A mockingbird tries to warn Jem and Scout about the dangers they face during their walk past the Radley property on Halloween. And newspaper editor B. B. Underwood likens Tom's death to "the senseless slaughter of songbirds..."