they shoot at tin cans, but he knows his children. He tells them to shoot bluejays if they want (and if they can hit them) but reminds them that “it's a sin to kill a mockingbird.”

Scout doesn't know what her father means. She has never before heard him say that anything is a sin, so this statement surprises and confuses her. She goes to Miss Maudie for an explanation. Miss Maudie tells her, “Mockingbirds don't do one thing but make music for us to enjoy.” They don't ruin gardens or make inconvenient nests. They just “sing their hearts out for us.” They are innocent, and they fill the world with beauty. No one should kill a creature like that. In fact, it is an act of wickedness to do so.

After Miss Maudie's explanation, Scout understands to a point, but Atticus's words don't really reach deeply into her mind until Boo Radley rescues her and Jem from Bob Ewell, killing Ewell in the process. Atticus and Mr. Tate decide that they will simply say that Ewell fell on his knife. They do not want to expose Boo to the attention that he will receive if they reveal his part in the incident. They realize that the community's gratitude will overwhelm Boo. Atticus asks Scout if she understands. That's when the idea of the mockingbird rises to the top of her mind. “Well, it'd be sort of like shootin' a mockingbird, wouldn't it?” Scout asks her father. Boo Radley does what he can to spread beauty throughout the world, even though he is a recluse. Recognition and attention would kill his spirit just as surely as a bullet from an air rifle would kill a mockingbird.

Atticus's famous life lesson takes place towards the beginning of chapter 10 and can be found on page 103 of the Fifteith Anniversary First Harper Perennial Modern Classic edition of To Kill a Mockingbird. This quote is significant to the story because it underscores the main theme of the novel, which concerns the importance of defending innocent beings and protecting them from harmful, prejudiced individuals and institutions.

In the story, several notable characters are considered symbolic mockingbirds and rely on others for protection. Bluejays are also symbolically significant and represent ignorant racists like Bob Ewell. In the quote, Atticus is giving his children permission to shoot all the bluejays they want but instructs them never to kill a mockingbird.

Atticus is metaphorically giving his children permission to challenge and stand up to dangerous racists and criminals. He is also teaching them the importance of protecting innocent, defenseless people, who rely on others for assistance. Atticus demonstrates this important life lesson by defending Tom Robinson against the racist Ewells.

Scout matures as the story progresses and applies her father's lesson about mockingbirds to Boo Radley's precarious situation. Scout understands Sheriff Tate's reasoning for concealing Boo's heroics and concludes that exposing him to Maycomb's nosy community would be like "shootin‘ a mockingbird."

In Chapter 10, paragraph 7, page 94 of the Warner Brothers edition, Atticus gives his children permission to shoot blue jays if they can hit them, but they must not kill mockingbirds.

Interestingly, Les Line, an avid bird watcher who quotes this passage from To Kill a Mockingbird, blames American ornithologist, naturalist, and painter John James Audubon partly for the notorious reputation that blue jays have.

His stunning plate of three glorious specimens sucking eggs “pilfered from the nest of some innocent dove or harmless partridge” was widely reproduced on calendars handed out by insurance companies in the mid-20th century, helping to foment blue jay hatred. (Audubon)

While these smart birds know how to avoid the trip on a trap filled with sunflowers after experiencing it once, they are actually helpful to nature because they disperse acorns and beechnuts from North American forests. 

It does seem somewhat out of character for the kind-hearted Atticus, who himself is reluctant to use a gun on even a rabid dog, to condone the killing of any creature.
Of course, Harper Lee wrote her novel before blue jays fell under the protection of the federal Migratory Bird Treaty Act, and killing blue jays was probably acceptable in the culture of Southern Alabama, especially in the 1930s. Nevertheless, Atticus's words about the blue jays do seem to mitigate the sincerity of the concern for mockingbirds, those grey birds who mimic the songs of other birds prettily, but who also often mimic the sounds of insects and amphibians loudly and in rapid succession. So they are not exactly quiet themselves.

But To Kill a Mockingbird is a fictional novel, so the mockingbird makes a convenient symbol, one to which Tom Robinson and Boo Radley can be compared in their innocence. Also, it is one that Atticus can use with the children in order to teach them to be kind to innocent creatures be they bird or man.

In Chapter 10, on page 119 of the Grand Central Publishing edition of the novel To Kill a Mockingbird, Atticus says,

"Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird." (Lee 119)

Jem and Scout had recently been given air rifles from their Uncle Jack, and Atticus refuses to teach them how to shoot their guns. After Atticus tells them it is a sin to kill mockingbirds, Scout mentions that it was the only time she's ever heard her father say it was a sin to do something. Miss Maudie elaborates on why Atticus told them it was a sin to kill mockingbirds. She tells the children that mockingbirds do nothing but make music for people to enjoy all day, and do not eat up people's gardens or nest in their corncribs. Mockingbirds are symbolic of innocent beings throughout the novel. Tom Robinson and Boo Radley are symbolic mockingbirds because they do not harm anyone and only bring joy to those around them. Atticus' lesson that it is a sin to kill a mockingbird alludes to the belief that it is wrong to harm innocent human beings. This is an important lesson in Jem and Scout's moral development that teaches them to treat all beings with respect and tolerance.

One of the most important and memorable quotations in the novel, this is spoken by Miss Maudie to Scout in Chapter 10 (page 90 in my old paperback version from 1982). Scout has remembered Atticus's advice to his children about it being okay to shoot tin cans and even blue jays with their new air rifles, but never a mockingbird. It was a "sin to kill a mockingbird," and Maudie explained to Scout that unlike blue jays, mockingbirds were not harmful to humans or their "gardens" and "corncribs." Instead, they only "sing their hearts out" for people to enjoy. The innocence of the mockingbird is extended symbolically to many of the human characters, including Boo Radley, Tom Robinson, and most of the children in the novel.