Most scientists now agree that birds are living . The first strong came from Archaeopteryx, a fossil found in Germany in 1861, had feathers a bird but teeth and a long bony like a reptile. Since the 1990s, feathered dinosaur fossils have been in China. They that feathers appeared long flight, perhaps for warmth or .

If feathers did not first evolve for flight, what were they for? Fossils suggest that the earliest feathers were simple, hair-like filaments, more like fuzz than the stiff flight feathers of modern birds. Such fuzz would have been useful for keeping small animals warm, much as fur does for mammals.

Later, more complex feathers appeared, including ones with a central shaft and branches on each side. Some of these were probably used for display. In 2010, scientists studied tiny structures called melanosomes, which hold pigment, in fossil feathers. By comparing their shapes with those in living birds, they were able to suggest the colors of some dinosaurs. One small dinosaur, Sinosauropteryx, appears to have had a striped tail of orange-brown and white.

Feathers may also have helped some dinosaurs keep their eggs warm. Fossils of the dinosaur Citipati have been found sitting on nests in the same position as brooding birds, with their arms spread over the eggs.

Only later, it seems, did some feathered dinosaurs begin using their feathers to glide and eventually to fly. Evolution often works this way: a feature that develops for one purpose is later adapted for another.