Birds are warm-blooded, backboned animals with feathers, and belong to the class Aves. The debate as to whether birds are closely related to dinosaurs has been going on for some time. The debate is very important because if birds are proved to be closely related to dinosaurs, it lends evidence to the argument that dinosaurs were warm blooded.

For many years of the twentieth century, however, the fact that dinosaurs did not appear to have the collar bones from which birds derive their wishbone was thought to be proof that birds are not close relatives of the dinosaurs. More recent, close analysis of the remains of one of the first birds, Archaeopteryx, has shown that there are many similarities between Archaeopteryx and small theropod dinosaurs like Deinonychus and Compsognathus, including the presence of a wishbone-like structure.

In the past several years, new discoveries have added fresh evidence to the link between dinosaurs and birds. Sinosauropteryx was a small early Cretaceous dinosaur whose remains were discovered in the Liaoning Province of northeastern China in 1996. This dinosaur was covered in filaments that may have been the forerunners of feathers. A year later, a second dinosaur called Caudipteryx was also found in Liaoning Province. Caudipteryx shows filaments more closely related to the feathers of birds, although it is unlikely that this dinosaur could fly.

Unenlagia was a 90 million year old theropod dinosaur whose remains were found in the badlands of Patagonia in 1996. Unenlagia could move its shoulder joint in the same way that birds raise their forelimbs to flap their wings. This dinosaur was not a bird, but it shows that dinosaurs were evolving characteristics that would be important for flight.

The evolution of the characters that make flight possible - feathers, lightweight bones and joints that could articulate a flapping motion and provide control during flight, probably all came about for different reasons. Once the ability to fly had evolved however, refinements in the efficiency of flight could be further evolved.