Meat-eating two-legged dinosaurs slowly acquired bird-like features over tens of millions of years, according to a new family tree of the creatures produced by Scottish researchers.
Once basic pieces of the bird "kit" - such as feathers, wings and wishbones - were assembled, avian evolution took off.
But there was no single "missing link" between ancient dinosaurs and modern birds, and every bird, they say, is descended from Tyrannosaurus rex.
T rex was a theropod, a large family of dinosaurs that stood on two legs and included the largest land-dwelling carnivores that ever lived.
All birds are the descendants of theropods.
Dr Steve Brusatte, from Edinburgh University's School of GeoSciences, who led the research, said: "There was no moment in time when a dinosaur became a bird, and there is no single missing link between them.
"What we think of as the classic bird skeleton was pieced together gradually over tens of millions of years.
"Once it came together fully, it unlocked great evolutionary potential that allowed birds to evolve at a super-charged rate."
The scientists analysed the anatomical make-up of more than 850 body features in 150 extinct species and combined the findings with statistical techniques to build up the family tree.
The results, published in the journal Current Biology, confirmed that the emergence of birds 150 million years ago was a gradual process, as some dinosaurs became more bird-like.
They support a theory first proposed in the 1940s that said the emergence of new body shapes could trigger an evolutionary surge.
Co-author Dr Graeme Lloyd, from Oxford University, said: "Our study adds to a growing number of works that approach this problem from different angles, but all seem to confirm that the origin of birds was a truly special event in history.
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