Atticus makes this statement only once, in Chapter 10 of the novel. It precedes his taking up a gun once again and killing the mad dog that threatens his neighborhood. The mockingbird motif can be found throughout the novel, beginning with Atticus's own name, Finch--a type of harmless songbird not unlike the mockingbird; but the word mockingbird only appears a few times. It can be found again in Chapter 10 when "the mockingbirds were silent" as the rabid dog stalked the street. It is mentioned just before the jury returns its verdict in the trial of Tom Robinson as Scout compares the scene to the day Atticus killed the dog:
... exactly the same as a cold February morning, when the mockingbirds were still... (Chapter 21)
Scout compares Boo Radley to a mockingbird at the end of Chapter 30 when she agrees with Sheriff Tate's decision to call Bob Ewell's death self-inflicted, eliminating the need for Boo to face an inquest or trial. She tells Atticus
"Well, it'd be sort of like shootin' a mockingbird, wouldn't it?" (Chapter 30)
B. B. Underwood conjures up images of the mockingbird when he compares Tom's death "to the senseless slaughter of songbirds by hunters and children" in his editorial (Chapter 25).
Miss Maudie also mentions the mockingbird when she explains Atticus's comment to Scout (in Chapter 10), about how "Mockingbirds don't do one thing but make music for us to enjoy."