In chapter 10 Atticus tells his children that "it's a sin to kill a mockingbird " what reason does he give for saying this?

The mockingbird represents a sense of innocence. It merely sings and brings joy to people.

"Atticus said to Jem one day, 'I'd rather you shoot 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.'