In Harper Lee's To Kill A Mockingbird, Atticus tells his children that "it's a sin to kill a mockingbird." What reason does he give for saying this?

In Harper Lee's To Kill a
Mockingbird, the kids get air-rifles from Uncle Jack for Christmas. Uncle
Jack gives them basic instruction about how to use the air-rifles, telling them that
Atticus isn't much interested in guns. (This is an ironic statement based on the need of
Atticus' excellent marksmanship when the threat of a rabid dog enters the
neighborhood.)

Atticus gives the children instructions
about what they can and cannot shoot.

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I'd rather you shot tin cans in the backyard, 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.

(*Killing
bluejays probably reflects the predatory nature of bluejays to attack and kill other
birds.) It is actually Miss Maudie that explains Atticus' stand on
mockingbirds.

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Mockingbirds don't do one thing but make music
for us to enjoy. They don't eat up people's gardens, don't nest in corncribs, they don't
do one thing but sing their hearts out for us. That's why it's a sin to kill a
mockingbird.

This reflects a
central theme of the story: symbolically, Tom Robinson and Boo Radley are seen as
mockingbirds in the story: neither one has ever caused harm to anyone, and yet they are
both victimized. Tom is tried and convicted for a crime he did not commit, and Boo has
been victimized by his family—first by his father and then his brother, so he is nothing
but a shadow of the young man he once was, but almost a
ghost.

The idea behind protecting mockingbirds is that no
harm should come to those who cause no harm.