Boo Radley is a metaphorical mockingbird because he sews Jem's torn pants for him, he hides little gifts in a tree hole for the children, and he saves Jem's life; he harms no one without reason.
The mockingbird are a song bird that does not do any damage to other animals or to man. As Miss Maudie says, "They don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird." Much like the mockingbird, Boo Radley stays inside his house, not bothering anyone. For this reason, Atticus tells the children to leave him alone, and Miss Maudie calls upon their sympathies, explaining that the Radley house is a "sad house" (Ch.5). Long ago Boo was punished for being with some young men who disturbed the peace and did a few other things. The others went to the reform school, but Boo was made to stay at home by his stringent father. He has been inside ever since his teen years, and has harmed no one.
Here is more textual evidence supporting Boo's being a metaphorical mockingbird:
- After Scout runs past the Radley place on her way home from school one day, she notices some foil stuck in the Radley tree that is near the street. Approaching the tree, she finds chewing gum there Later, there are other gifts, such as two Indian head pennies. Later, Jem rolls Scout inside a tire inadvertently into the Radley yard. But, when she is dumped from the tire, Scout thinks she hears laughing coming from inside the house, as though someone has been watching.
- In Chapter 6, the children decide to peek into the window of the Radley house. But, when Mr. Nathan hears them, he steps onto the back porch with a rifle; the children see a shadow that raises its arm, then drops it. Scout flees in terror and the boys follow her, but Jem gets his pants caught on a barbed wire fence, so he has to step out of them. When he sneaks out later that evening and retrieves them, they have been mended, and they are folded over the fence. (Boo has repaired them)
- In Chapter 28 Scout and Jem are attacked by a drunken Bob Ewell, and Boo Radley saves them by attacking Ewell after he breaks Jem's arm. Then, Boo carries Jem across the yard to Atticus who is on the front porch. After a while, Scout takes Mr. Arthur in to see Jem and be reassured that he is all right; afterwards, she walks him back to the Radley house.
Sheriff Tate tells Atticus that he will report that Ewell fell upon his own knife because he has never heard that it is unlawful for a citizen to make every effort to prevent a crime. Further, he tells Atticus,
"To my way of thinkin', Mr. Finch, taking the one man who's done you and this town a great service and' draggin' him with his shy ways into the limelight--to me, that's a sin."
Scout reassures her father, "...it'd be sort of like shootin' a mockingbird"