How do we know birds are living dinosaurs?

Because feature after feature of a bird's body turns up in dinosaurs. The present scientific consensus is that birds are a group of maniraptoran theropod dinosaurs that originated during the Mesozoic. Fossils of more than thirty species of non-avian dinosaur with preserved feathers have been collected, including tiny ones like Microraptor, with long, vaned feathers on arms and legs forming wings. Yet paleontologists say the more important proof is the skeleton: the skull, a flexible wrist, a three-fingered hand, a wishbone and more are shared by birds and dinosaurs.

The idea is older than most people think. From 1868 Thomas Huxley compared the early bird Archaeopteryx with prehistoric reptiles and found it most similar to dinosaurs like Compsognathus. Then Heilmann, whose approach was extremely thorough, noticed that birds have a wishbone, a furcula made of fused collarbones, which had not yet been recognised in dinosaurs, so he looked for bird ancestors among more primitive reptiles, the 'thecodonts'. His book became a classic, and for the next four decades nearly all evolutionary biologists accepted its conclusions. Paleontologists now accept that clavicles and, in most cases, furculae are a standard feature of theropod dinosaurs.

The idea returned after 1964, when a new theropod was found in Montana, later named Deinonychus by John Ostrom. In the 1980s, cladistic analysis showed unequivocally that birds were a derived group of theropod dinosaurs. In 1996 Chinese paleontologists described Sinosauropteryx as a new genus of bird, but it was quickly recognised as a theropod dinosaur closely related to Compsognathus, and the first dinosaur outside birds and their close relatives found with evidence of feathers.

A small minority still disagrees. Known by the acronym BAND, Birds Are Not Dinosaurs, they hold that birds are more closely related to earlier reptiles than to dinosaurs. The consensus remains that birds are dinosaurs.