what are the adaptations unique to animals in class Aves only and are not present in any other class of animals?

Answer 1

1. Feathers. The so-called feathered dinosaurs are a hoax. No dinosaur has ever been found with feathers and any filament that share homology with avian feathers. Filaments found around the outline of dinosaurs like Sinosauropteryx are most likely collagen fibers. The only known reptile that has feathers homologous to avian feathers is Longisquama insignis, a Triassic reptile from Central Asia. 

2. avian style 4 chambered heart. Mammals also have 4 chambered hearts but they are different anatomically than the avian hearts. 

3. Supracoracoideus flight muscle. Not found in all birds (e.g. not in Archaeopteryx), but it allows a bird to take off flying from the ground or from the surface of the water.  Bats and pterosaurs do not have it. 

4. Reversed hallux or first toe. The first toe is pointed in the opposite direction of their other toes, allowing birds to grasp branches with their feet. 

5. Avian style cranial kinesis. The upper jaw can move up and down relative to the skull. A feature not found in other classes of vertebrates. such as mammals, reptiles fish or amphibians. 

Answer 2

Flight feathers.

Fusion of finger bones to support the leading edge of the wing

  That's all I could think of.  The problem is that some of their other features may or may not have been shared with dinosaurs or pterosaurs or both.

Other, possibly shared features:

"Hollow" bones.

Air-sac bones that are part of the respiratory system.

Partial one-way breathing (in air).

"Backward" knees (in extant birds only;  there's an extinct clade of birds that had knees that were not "backward")

Answer 3

Wings...insects have them too

toothless beaks... well there's the platypus

feathers?