In 2003, a group of Chinese researchers excavated six fossils of flying dinosaurs with wings on their hind legs, dubbing them Microraptors.
"When we published that ten years ago there was some suspicion whether the fossil was faked," Xing Xu from Linyi University in Shandong province said.
Now Xu and colleagues have described another find supporting their claim, a series of bird skeletons that have large feathers on their hind legs, NewScientist.com reported Thursday.
They were likely to have been utilized for flight rather than display, he said.
The fossil specimens lived in the Early Cretaceous, about 120 million years ago, and represent several primitive bird groups.
The findings suggest the branch on the tree of life that contains modern birds was populated in its early period by animals that look as if they had four wings, Xu said.