The researchers believe that miniaturisation and the development of bird-like traits had a joint influence on the evolution of the dinosaurs into today's birds.
Professor Michael Benton, from the University of Bristol's school of earth sciences, said: "This study means we can't see the origin of birds as a sudden or dramatic event, with a dinosaur becoming a powered flyer overnight.
"The functions of each special feature of birds changed over time - feathers first for insulation, and later co-opted for flight; early reductions in body size perhaps for other reasons, and later they were small enough for powered flight; improvements in sense of sight and enlargement of brain - even a small improvement in these is advantageous.
"So perhaps it's a long-term trend associated with deputation to a new set of habitats, in the trees, to avoid predation, and to exploit new food resources."
