Meat-eating two-legged dinosaurs slowly acquired bird-like features over tens of millions of years, a new family tree of the extinct creatures has shown.
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.
Every bird living today is a distant relative of Tyrannosaurus rex, the fearsome "king of the dinosaurs".
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 the University of Edinburgh'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 over time.
They support a theory first proposed in the 1940s that said the emergence of new body shapes in groups of species could trigger an evolutionary surge.
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