What characteristic is unique to birds among vertebrates?

Feathers and wings are indeed a characteristic unique to birds among vertebrates. While some other vertebrates may share traits such as laying amniotic eggs or having the ability to maintain their body temperature in varying conditions, feathers and wings are distinctly found in birds. Feathers serve several vital functions, including insulation, waterproofing, and, most notably, enabling flight. Wings, which evolved from the forelimbs of their ancestors, are specialized adaptations that contribute to their capability of flight, providing birds with the ability to take advantage of aerial habitats and escape from predators. This combination of feathers and wings is a hallmark of avian evolution, making it a defining trait that sets birds apart from all other vertebrate animals.