Scientists have created the most comprehensive family of carnivorous dinosaurs to date, to discover key details of how birds evolved from them.
The familiar anatomical features of birds, such as feathers, wings and wishbones, all first evolved piecemeal in their dinosaur ancestors over tens of millions of years.
Yet what led to the thousands of avian species that exist in the modern day was an evolutionary explosion once a fully-functioning bird body was complete, causing a rapid increase in the rate at which birds evolved.
Dr Steve Brusatte, of the University of Edinburgh, who led the study, 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."
To understand this timeline, researchers at the University of Edinburgh examined the evolutionary links between ancient bird and their closest dinosaur relatives.
An analysis of the anatomical make-up of more than 850 body features in 150 extinct species brought together a detailed family tree.
Based on their findings from fossil records, researchers say the emergence of birds some 150 million years ago was a gradual process, as some dinosaurs became evermore bird-like over time.
This makes it very difficult to draw a dividing line on the family tree between dinosaurs and birds.
Findings from the study, though, support a controversial theory proposed in the 1940s that the emergence of new body shapes in groups of species could result in a surge in their evolution.
"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 Earth history," said researcher Dr Graeme Lloyd, of the University of Oxford.
"It is particularly cool that evidence from the fossil record shows how an oddball offshoot of the dinosaurs paved the way for the spectacular variety of bird species we see today."
Recent research has made more headway into the evolution of birds, including the discovery of Siberian fossils of a two-legged, feathered dinosaur dating back roughly 160 million years ago. The National Geographic reported that the finding suggested all dinosaurs were covered in feathers at one point in history.
The study is published in the journal Current Biology.