There's no single "missing link" creature in the evolution of some dinosaurs into birds, researchers say, but rather a burst of evolution that produced thousands of bird species.

The anatomical features that define birds, including wings, feathers and wishbones to which wings are anchored, came about in a piecemeal fashion in some dinosaur bird ancestors over tens of millions of years, a study suggests.

However, once all the parts were in place, there was an evolutionary burst that led to the abundance of bird species in the world today, researchers say.

A study of fossils led by the University of Edinburgh yielded evidence that the emergence of birds around 150 million years ago was a long and gradual process as some dinosaur species slowly evolved to become more bird-like with time.

The researchers analyzed more than 800 body features displayed by 150 extinct bird species and close dinosaur relatives to construct a family tree that showed the first birds would have been difficult to distinguish from their nearest dinosaur ancestors.

That makes a hard dividing line in a family tree of birds and dinosaurs almost impossible to draw, researchers say.

"There was no moment in time when a dinosaur became a bird, and there is no single missing link between them," says Edinburgh study leader Steve Brusatte. "What we think of as the classic bird skeleton was pieced together gradually over tens of millions of years."

The new findings support a theory, dating to the 1940s, which proposed that an evolutionary novelty -- in this case, flight -- can result in a rapid explosion of new species taking advantage of novel environmental niches.

"Once [the bird form] came together fully, it unlocked great evolutionary potential that allowed birds to evolve at a super-charged rate," Brusatte, a paleontologist, says.

While the label "bird" is rather arbitrarily applied, the researchers say, the feathered fossil Archaeopteryx probably can be considered the first of the group.

"It is particularly cool that it is evidence from the fossil record that shows how an oddball offshoot of the dinosaurs paved the way for the spectacular variety of bird species we see today," says study co-author Graeme Lloyd of the University of Oxford in England.

It's not altogether clear why birds were so successful and spawned the thousand of modern species, the researchers say, but Brusatte says it's possible the warm-blooded, fast-moving creatures were capable of surviving the conditions that led their non-avian dinosaur ancestors to die out.