The title of Harper Lee's novel To Kill A Mockingbird has encouraged discussion over its symbolism and importance. In the book, Atticus and Miss Maudie make reference to it. Atticus allows his children to have air-rifles but is reluctant to teach them how to use them and instead their Uncle Jack teaches them. Atticus does, however, caution the children to "remember it’s a sin to kill a mockingbird" (chapter 10). They can practice on tin cans and even shoot bluejays but because mockingbirds do not steal other birds' eggs or ruin crops or do anything harmful, it would not be right to shoot at them with their air-rifles. This comment has quite an impact on Scout as her father does not ordinarily talk about committing "a sin" because he does not judge others; he is always encouraging his children to consider other people's opinions. So Scout questions Miss Maudie, who she knows will always be honest with her.
The full significance of Atticus's comment as it relates to the title will only be revealed later, but Miss Maudie reinforces it and tells Scout how true her father's words are. Miss Maudie tells Scout that mockingbirds sing sweetly, "singing their hearts out for us," and as such, it would be cruel and unnecessary to hurt them. This allows the reader to consider the unfairness of Tom Robinson's trial, which should not even be taking place. Just as shooting a mockingbird would never become necessary because it is assured that they will never do anything to contradict their very sweet nature, so Tom has never done anything to suggest that he is capable of the crime of which he stands accused.
It should not be necessary to prove Tom's innocence because he has never done anything to make others doubt his integrity, whereas the scorned Ewell family, who are at the center of the charges against Tom, is considered "trash" and defies everything that the community of Maycomb apparently stands for. Therefore, Tom Robinson is the ultimate mockingbird as his kindness to Mayella Ewell results in his being effectively shot down by the community where racism is so entrenched that it does not see its own hypocrisy or its contribution to the tragedy that is unfolding.
There are other characters who are innocent and are victims of their circumstances such as Boo Radley and Dill. They both suffer because of other people's selfishness and insecurity and they also contribute to the significance of the title.
Atticus explains to Scout that "it is a sin to kill a mockingbird" in one of their many discussions. His point is that because the bird is defenseless and innocent of any wrong doing, it would not be right to take its life. It did not deserve to be killed. It deserved to live. This was said to appeal to a child's natural sense of right and wrong.
The same reasoning was used in Atticus' defense of Tom Robinson. If he was innocent, as Atticus believed him to be, and since he was for all practical purposes as defenseless as the proverbial Mocking Bird, it would be wrong to allow his life to be taken as well.
The appeal to logic and the goodness of a child's heart is a very touching and effective strategy used by Lee to "win over" the reader.