Why does Atticus consider it a sin to kill a mockingbird and not a blue jay?

In Chapter 10, Atticus tells Jem and Scout that they can shoot all the blue jays they want if they can hit them, but it is a sin to kill a mockingbird. Scout asks Miss Maudie why it is a sin to kill a mockingbird, and Maudie tells her that mockingbirds do nothing but sing joyfully. She also says that mockingbirds don't nest in anybody's corncribs or dig up their gardens. Mockingbirds are essentially innocent beings that do nothing to bother anybody. However, blue jays are notorious for ruining people's gardens and plundering other bird's nests. Blue jays are considered a nuisance which is why Atticus allows his children to shoot them. Metaphorically speaking, mockingbirds represent innocent people like Boo Radley and Tom Robinson. Blue jays symbolically represent malicious people like Bob Ewell. Atticus' comment represents his feelings about protecting innocent beings and punishing those who attempt to harm others.