The earliest birds did not have strong enough feathers to take to the air by flapping their wings and were gliders at best, researchers said on Thursday.
While modern birds have feathers with a strong central shaft that is hollow to reduce weight, the earliest-known bird Archaeopteryx and another ancient ancestor had feathers that were much thinner and weaker.
Robert Nudds of the University of Manchester and Gareth Dyke of University College Dublin calculated in a report in the journal Science that even if their feather shafts were solid, they would still have been barely strong enough to allow gliding.
Archaeopteryx lived in the late Jurassic period, about 140 million years ago, and Confuciusornis in the early Cretaceous, around 100 million years ago.