Which evolutionary adaptation is primarily associated with the class Aves?

Feathers are a defining characteristic of the class Aves, or birds, and are primarily associated with their ability to fly. This evolutionary adaptation provides crucial benefits, including enabling flight, which allows birds to escape predators, search for food, and migrate over great distances. Feathers also play a role in insulation, helping birds maintain their body temperature, and in some cases, they contribute to mating displays and camouflage.

While swimming is a trait seen in some bird species, it is not a defining characteristic of all birds, as many do not swim at all. Similarly, fur is a characteristic associated with mammals, not birds, and scales are typically found in reptiles and some fish. Thus, feathers are the unique evolutionary adaptation that most clearly defines the class Aves and distinguishes them from other animal classes.