In "To Kill a Mockingbird" how and why does Harper Lee use the mockingbird as a key image in this novel?
The mockingbird is a songbird that sings beautiful songs, and Harper Lee uses it as a symbol of both Tom Robinson, and to a smaller degree, Boo Radley. No, Tom and Boo don't go around singing beautiful songs in the trees. :) The mockingbird is a symbol of innocence, and Scout mentions how after she and Jem got air-rifles, Atticus warned them not to shoot the mockingbirds, because "it's a sin to kill a mockingbird," meaning, a bird who is just innocently singing its song should not be shot or harmed. It is innocent, harmless, and just going about its business, and to harm a helpless and innocent creature is a sin. Later, B. B. Underwood makes a direct comparison between Tom and a mockingbird. He
"likened Tom’s death to the senseless slaughter of songbirds by hunters and children,"
meaning, shooting Tom, because of its senseless waste and cruelty, was like shooting an innocent songbird.
Tom represents a mockingbird because he was an innocent man who was unjustly condemned to jail and then killed, just for show and man's senseless pride. Boo represents a mockingbird because he is more or less just a man living his life, who is put on display for people's curiosity and fascination, just like a prize bird would be displayed. The mockingbird works well to symbolize the senseless harm that some cause in the lives of others. I hope that those thoughts help some; good luck!