AVES is really a fancy word for BIRD. The answer to this question depends a bit on what you mean by the word "unique." If by unique you mean "unusual when put together as a whole," we could add some things to the list right off the bat:
- Warm blooded
- Egg laying
If, on the other hand, by "unique" you mean exclusive, the list gets shrunk some. Here are three things that seem to only relate to class AVES:
- Feathers -- Allowing the bird to fly and stay warm
- Bills -- Birds don't have teeth, but rather use hard bills
- Furcula -- Birds have a special "wishbone" in their chest that keeps it from being squished while flying.