Birds evolved from small, feathered theropod dinosaurs during the Jurassic period, around 150 million years ago. Archaeopteryx, one of the earliest known birds, showcases traits of both dinosaurs and modern birds, highlighting this transition. Over time, these early avians developed more specialized features like fused bones, lightweight bodies, and advanced respiratory systems for flight. During the Cretaceous period, birds diversified significantly. After the mass extinction 66 million years ago, which wiped out non-avian dinosaurs, birds thrived, evolving into the vast array of species we see today, from tiny hummingbirds to majestic eagles.