Tom is referenced as a mockingbird in chapter 25. Mr. Underwood writes an article in The Maycomb Tribune that compares Tom's death to the senseless killing of songbirds. His point must have been that Tom did not deserve to be shot 17 times in his alleged escape. This is comparable to hunters or even children using guns to kill good birds for sport. In fact, in 1918, Teddy Roosevelt helped craft an act that defended many wild birds and songbirds, it would have been in effect during this time.
In the last chapters, Scout is given the opportunity to assess why Heck Tate doesn't want to turn Boo Radley in for the death of Bob Ewell. Even though many in the town would praise his work, it's not like him to want any piece of the spotlight. Atticus asks her if she can understand that Bob Ewell fell on his knife and why the situation is the way it is. She replies that they have to otherwise it would be sort of like shootin' a mockingbird.