During which period did the first birds appear?

The first birds appeared during the Jurassic period, which is significant because it marks an evolutionary milestone in the transition from dinosaurs to modern birds. Archaeopteryx, often cited as one of the first and most famous birds, lived in the late Jurassic and displayed both avian and reptilian features. This transitional fossil demonstrates key characteristics such as feathers, which are essential for flight, alongside other traits that are common to theropod dinosaurs.

During the Triassic, the early ancestors of both birds and mammals emerged, but true birds did not exist yet. The Cretaceous period is notable for the diversification of bird species and the dominance of birds after the mass extinction event at the end of this period. The Paleozoic era predates the existence of birds entirely, focusing on the evolution of invertebrates and early fish. Thus, the Jurassic period is correctly identified as the time when the first true birds appeared.