Avian dinosaurs, commonly known as birds, are a group of feathered reptiles that evolved from theropod dinosaurs during the late Jurassic period. They are characterized by features such as feathers, hollow bones, and a beak, which help them adapt to flight. The earliest known avian dinosaur is Archaeopteryx, which lived around 150 million years ago and displayed both dinosaur and bird traits. Today, avian dinosaurs are the only surviving lineage of the larger group known as Dinosauria. They have diversified into thousands of species, occupying various habitats worldwide. Their evolutionary success is attributed to their ability to fly, which allows them to escape predators and find food more efficiently.