Mr. Underwood's editorial in the Macon Tribune after the shooting of Tom Robinson is comparable to Atticus's advice to Jem and Scout when they receive their air-rifles because both bring up the mockingbird motif.
In Chapter 9, after their father Scout describes her father's attributes; she explains that he gave up hunting because he felt that he had an unfair advantage over animals because of his keen eyesight. He cautions his children after having given them air rifles, saying that he prefers that they not shoot mockingbirds because, unlike bluejays, they do not tamper with gardens, nest in corncribs, eat the eggs of other birds, or bother anything.
When Mr. Underwood rails in his editorial against the bias of the community that has been directed gratuitously directed at the innocent Tom Robinson, he writes that
...it was a sin to kill cripples, be they standing, sitting, or escaping. He likened Tom's death to the senseless slaughter of songbirds [mockingbirds and such] by hunters and children....
Mr. Underwood, like Atticus, realizes that the white community has had an unfair advantage over the innocent "Negro," Tom Robinson, just as humans with guns have an unfair advantage over mockingbirds. In each case the cruelty is gratuitous.