Final answer:
Feathers are a true defining characteristic of birds (class Aves), which are endothermic amniote tetrapods that evolved from theropod dinosaurs. Their feathers are not only for flight but are also modified reptilian scales, which contribute to the birds' wide range of adaptations and ecological success.
Step-by-step explanation:
True. Feathers are a shared derived character that define birds and differentiate them as a distinct and separate class of tetrapods.
Feathers are indeed fundamental to the class Aves, or birds, and are considered a defining feature of this group, but their evolutionary origins predate birds themselves. Birds evolved from theropod dinosaurs around 150 million years ago, and recent fossil discoveries suggest that feathers might have been present in their common ancestors with other dinosaurs. Feathers and bird wings are adaptations for flight, which is a key aspect of avian ecology and contributes to their success.
Birds are endothermic amniote tetrapods characterized by bipedal locomotion, egg-laying, and other unique adaptations besides feathers, such as pneumatic bones and a high metabolic rate. Although birds are a modern class of vertebrates, their evolutionary history reflects deep ties to extinct dinosaur lineages.