When Atticus says "shoot all the bluejays you want" but "it's a sin to kill a mockingbird," what does he mean and where do we see examples of it in...

Atticus doesn't fully explain his rationale for the advice
he gives Jem that serves as the title of the Harper Lee novel. Jem and Scout have
received air rifles as Christmas presents from Atticus, and he tells Jem
that

"I'd
rather you shot at tin cans in the back yard, but I know you'll go after birds. Shoot
all the blue jays you want, if you can hit 'em, but remember it's a sin to kill a
mockingbird." 

It is Miss
Maudie who explains to Scout that mockingbirds are harmless creatures
who

"... don't
do one thing but make music for us to enjoy... (and) sing their hearts out for us.
That's why it's a sin to kill a
mockingbird."

Atticus'
implication is that shooting cans does not involve the killing of God's creatures; but
if it's kill you must, shoot at blue jays (which are harmful, eating fruits and other
crops). Atticus unexpectedly takes his own advice later in Chapter 10 when he is forced
to fire a gun for the first time in decades: It is not an innocent, harmless mockingbird
he shoots, but a dangerous and possibly deadly mad dog which threatens the lives of his
neighbors.