A mockingbird is a long-tailed thrush like songbird with grayish plumage, found mainly in tropical America and noted for its mimicry of the calls and songs of other birds. What we need to point out on here is that fact that this bird is a song bird and likes to mimic and copy other birds. Now if we go to the first part where Atticus talks about a mockingbird he says "Shoot all the blue jays you want, if you can hit ‘em, but remember it’s a sin to kill a mockingbird." Now why is it a sin to kill a mocking bird? Atticus answer this in the nest line when he says "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.” So theres the answer, but there is still one think that I don't quite understand. Why are mockingbirds so precious if they just copy other birds, Atticus never mentioned this in the story.
Now, for the most part we know that a mockingbird is a innocent person and that dies or it could also mean a person whose innocence dies. This opens an entire window of people who it could possibly be. Scout is growing old and learning about all these terrible things that happen in the world; she is losing he innocence. Atticus is being accused by his own town for taking this case even though he knew he had to take it. Jem is growing up and slowly losing his innocence, but not as much as Scout. Tom Robinson was killed obviously for something has didn't do and he was trying to escape the prison to see his children (just like the poem about the caged in bird that we read). Mayella Ewell was only looking for a friend and she was sent to trial and he only friend was killed. The list goes on, there are so many people in the book that could possibly be the mockingbird because many lost their innocence and their death in the book.