Who is symbolized as a mockingbird in To Kill a Mockingbird?

The two symbolic mockingbirds are Boo Radley and Tom Robinson.

Atticus tells his children not to shoot mockingbirds when they get guns for Christmas.  He tells them that it is a sin to kill a mockingbird.  Scout is surprised, because her father never says anything is a sin.  She asks Miss Maudie to explain.  Miss Maudie tells her that mockingbirds are harmless, and only make nice music for us to enjoy.

The mockingbird thus becomes symbolic for someone who is victimized by society when he is really a gentle creature.  Tom Robinson is a victim because he is targeted by the Ewells because of his race.  He was trying to help Mayella.  For this, he was accused of rape and convicted.  In an editorial, Mr. Underwood compares him to a songbird, giving us our first symbolic mockingbird comparison. 

Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. He likened Tom’s death to the senseless slaughter of songbirds by hunters and children … (Ch. 25) 

Tom Robinson did not do anything to Mayella.  He was a scapegoat and target of fear. The people of Maycomb were just not ready to accept a black man’s innocence.  Knowing this, he killed himself by escaping prison.  He did not want to take his chances with an appeal. 

The second mockingbird is Boo Radley. He is a victim of society too.  Because he is different, he is ostracized.  He never even comes out of his house.  He is timid, shy, and reclusive.  Scout learns that he is not a monster, as the neighborhood believes, but just a gentle soul.  He rescues Scout and Jem from Bob Ewell.  Atticus asks Scout if she understands why they are keeping Boo’s involvement a secret. 

“Scout,” he said, “Mr. Ewell fell on his knife. Can you possibly understand?” Atticus looked like he needed cheering up. I ran to him and hugged him and kissed him with all my might. “Yes sir, I understand,” I reassured him. “Mr. Tate was right.  … Well, it’d be sort of like shootin‘ a mockingbird, wouldn’t it?” (Ch. 30) 

Telling everyone what happens would bring notoriety to Boo Radley, which he would not want.  They decide to pretend that Bob Ewell fell on his knife and died by accident while attacking the kids.  That way they can protect Boo and keep him out of it.  Bringing fame to such a shy man would be like killing a mockingbird.