In findings published on Thursday in PLoS Computational Biology, scientists from China's Tsinghua University and the Chinese Academy of Sciences reported bird flight may have evolved from wing-flapping by running dinosaurs. These findings support the hypothesis flight could have evolved up from the ground, in contrast to models in which active flapping flight evolved after gliding, a passive form of flight. 
The authors proposed that in Caudipteryx, from a mechanical perspective, two-legged running automatically induced wing flapping. The authors of the report used a mathematical model, a robot, and young ostriches to test their hypotheses.

For the study, the researchers chose Caudipteryx zoui, a dinosaur about the size of a peacock known to have feathered wings but not flight.  It weighed about ten lbs (five kg) and the researchers estimate it had a top speed of about 18 mph (8 m/s).  Using modal effective mass theory, the team conceptualized the dinosaur's body as a series of springs and other components, first as a computer model and then as a physical robot.  In both scenarios, running at medium speed produced vibrations making the animal's wings flap.Co-author Jing-Shan Zhao said, "Our work shows that the motion of flapping feathered wings was developed passively and naturally as the dinosaur ran on the ground[...] Although this flapping motion could not lift the dinosaur into the air at that time, the motion of flapping wings may have developed earlier than gliding."
They also attached sets of artificial wings to juvenile ostriches about the same size as Caudipteryx.  They found the faster each ostrich ran, the more it tended to flap, with larger wings providing more lift.
John Hutchinson, an evolutionary biomechanicist at London's Royal Veterinary College, questioned the findings, saying Caudipteryx might have dealt with the vibrations by holding its forelimbs tight against its body instead of flapping.  "However," he told Science News, "this study does lay groundwork that could be built upon and tested more rigorously. So perhaps it will inspire deeper insight in the future."
Although Caudipteryx is in the same evolutionary group as birds and other theropods, it lived millions of years after flying dinosaurs, such as Archaeopteryx, were already in the air.
While current consensus among scientists says birds as we understand them evolved from theropod dinosaurs, there are several competing ideas about how exactly flight developed in these vertebrates.  Some scientists believe the first fliers were tree-dwelling dinosaurs who could parachute and glide before they could fly, while some say flight grew up from the ground, from runners. 

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Scientific study suggests dinosaurs flapped their wings as they ran