Where to kill a mocking bird came from?

Atticus said to Jem one day, "I’d rather you shot at tin cans in the backyard, 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."

That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it. "Your father’s right," she said. "Mockingbirds don’t do one thing except make music for us to enjoy. They don’t eat up people’s gardens, don’t nest in corn cribs, they don’t do one thing but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird."

My ideas: 

When relating this to the book it may be suggested that the usage of a mockingbird is to highlight the loss of innocence. The symbolism of the mockingbird may be applied to key characters in the novel. Jem and Scout have their innocence taken from them by being exposed to racist trends, their fathers trial, and attempted murder of the two. Boo's parents robbed him from any childhood in which he may have had, and tom Robinson was trailed and died although innocent. The children also show a progression throughout the book, leaving their naivety and previous traits through the book.