...reported in Nature:

Instead of two wings, the first birds might have used four feathered limbs to stay aloft, according to research published today in Science.

Birdlike dinosaurs, such as Microraptor and Sinornithosaurus, are known to have had long, sturdy feathers on their hindlimbs. But until now, researchers were not sure whether the earliest birds had already abandoned this extra plumage when they emerged to take to the Cretaceous skies over 100 million years ago.

The researchers, led by Xing Xu, a palaeontologist at the Institute of Geology and Paleontology in Shandong, China, found evidence of feathers on the hindlimbs of 11 basal bird specimens (gathered from the Lower Cretaceous Jehol Group in China)3. On some of the individuals, these feathers appeared to be veined flight feathers that stood perpendicular to the leg bones, similar to those in the basal bird Archaeopteryx.

One specimen, attributed to the Sapeornis genus, had at least one hindlimb feather longer than 50 millimetres. Feathers on the feet were shorter, but were still more than 30 millimetres long.

"It is amazing that so many early birds had large leg feathers," Xu says. The first winged dinosaurs were discovered just 10 years ago, he notes. These findings "are important for both flight origin and feather evolution".

The hind limb plumage has been reported before, but this analysis "provides solid evidence for the existence of enlarged leg feathers on a variety of basal birds, suggest that extensively scaled feet might have appeared secondarily at an early stage in ornithuromorph evolution, and demonstrate a distal-to-proximal reduction pattern for leg feathers in avialan evolution."