How did small theropod dinosaurs evolve into birds?

Birds are descendants of small, bipedal carnivorous dinosaurs known as theropods. The transition from these dinosaurs to birds was not a sudden event but a gradual process driven by the evolution of key anatomical features, most notably feathers.12

Contrary to the idea that feathers evolved specifically for flight, fossil evidence—such as that from the Liaoning deposits in China—shows that primitive, filamentous structures existed in many non-avian theropods long before flight emerged. These structures likely served functions such as insulation, camouflage, species recognition, and courtship display. Some theropods even used their feathers to protect and warm their eggs while nesting.231

Over time, these simple filaments evolved into complex, vaned feathers. Simultaneously, theropods developed anatomical traits—such as hollow bones, a wishbone, and a unique wrist joint—that were originally adapted for terrestrial life and catching prey but later proved essential for flight. Eventually, in one specific lineage, these adaptations were co-opted for gliding and powered flight, leading to the emergence of birds.1342