What circumstances prompt Atticus to tell the children it is a sin to kill a mockingbird?

The children receive air rifles as Christmas gifts, which prompts Atticus to tell the children it is a sin to kill a mockingbird. Scout later questions Miss. Maudie about her father's admonition, and Miss. Maudie explains that mockingbirds bring nothing but pleasure with their beautiful songs.... they do no harm to anyone, and it is a sin to kill them.