How did birds evolve from theropod dinosaurs?

Modern birds are living dinosaurs that evolved from a lineage of small, carnivorous theropod dinosaurs more than 150 million years ago. While birds are often treated as a separate class, they are technically a specialized lineage within the reptile group Archosauria.123

The transition from theropods to birds involved the gradual acquisition of avian features:

• Feathers: Fossil evidence from the Liaoning deposits shows that feathers did not evolve for flight. Early filamentous structures likely served as insulation, camouflage, or for display. Over time, these evolved into complex, vaned feathers.42
• Anatomical Shifts: Birds inherited skeletal traits from theropods, including hollow bones, a wishbone, and a specific wrist joint that allowed for the side-to-side motion necessary for flight.526
• Physiology: Many theropods, including the ancestors of birds, were warm-blooded, a trait that supported the high energy demands of their active lifestyles.43

Genera such as Archaeopteryx represent this evolutionary bridge, possessing both dinosaurian features—like teeth and a long tail—and avian characteristics like wings and feathers.54