What does Atticus mean when he says they can shoot all the bluejays, but not the mockingbirds?

Atticus does not like guns, but he also does not expect his children to share his feelings.  Scout and Jem receive air rifles as gifts for Christmas.  Uncle Jack shows them how to shoot the air rifles.  Though Atticus does not like guns, he does offer advice on the topic:

"I'd rather you shot at tin cans in the back yard, but I know you'll go after birds.  Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird" (To Kill a Mockingbird, Chapter 10).

Though Atticus prefers his children not to shoot birds, he knows that they will.  He tells them that if they do choose to shoot birds, to go after bluejays.  He specifically instructs them not to shoot at mockingbirds.  He thinks that it is wrong to shoot mockingbirds because they are innocent.  They sing and do no harm, unlike some other birds.  This advice shows the method of parenting Atticus prefers.  He usually does not command his children not to do things.  Instead, he reasons with them.