This is an interesting question. You will be hard pressed to find evidence that Atticus is like a finch or Tom a robin (apart from their names). I am not saying that the bird imagery is not important, but we cannot press everything interesting into an insightful point.
That said, Tom Robinson is portrayed as a mockingbird. He is honest, innocent, and does good things for people, even Mayella Ewell.
If we make this connection (that Tom is a mockingbird), then it is a grave sin to harm him. This is exactly what happened. There were only a few people who would protect him, such as Atticus. So, we can say that a mockingbird was killed tragically.
We can also say that Tom, even though a mockingbird, was treated as a more common bird such as a robin or a bluejay. In other words, the tragedy is that an innocent man died. And this innocent man is one that people should protect, but he was left to be touched by evil.
The following words are spoke by Atticus, and they are symbolic.
Atticus said to Jem one day, “I’d rather you shot at tin cans in the back yard, but I know you’ll go after birds. Shoot all the bluejays you want, if you can hit ‘em, but remember it’s a sin to kill a mockingbird.”