US and Chinese researchers have found the oldest evidence of iridescent black feathers in Microraptor, a dinosaur the size of a small crow that perched in forest tree branches 130 million years ago.

Scientists think the glossy plumes may have helped the small, meat-eating dinosaur signal its good health and suitability as a mate to others, much the way fancy colors serve birds of our era.

The analysis was based on a fossil found in 2010 in northeastern China which shows the four-winged dinosaur skeleton and its long, tail-like hind limb embellished with a multi-plumed tip almost as long as its wing feathers.

Armed with more data than before about the creature's physical characteristics, and new ways to uncover the colors of feathers with scanning electron microscopes, scientists set about to uncover its hues.

By focusing on tiny elements of pigment in the fossil called melanomes and comparing them to those of modern birds, the team was able to tell that the Microraptor's feathers were black with a blue sheen.

"Just a few years ago it would have been inconceivable for us to have imagined doing a study like this," said Ke-Qin Gao, a coauthor of the study and researcher from Peking University in Beijing.

Microraptor was first described as a species in 2003, but the 2010 fossil offers the clearest image of it to date, and revealed the long hind limb that was previously unknown to science.

"What we have here is a new specimen of that species that shows the earliest evidence for iridescence in dinosaurs," said co-author Julia Clarke, an associate professor of paleontology at The University of Texas at Austin.

The fossil also sheds new light on how the dinosaur may have moved. Clarke said it is not likely that the creature soared like today's birds, but may have adopted more a flapping, hopping way of getting from branch to branch.

"If we were looking up into a Cretaceous forest I think perhaps they would appear not dissimilar to a crow," Clarke told AFP.

"You can imagine kind of glints of this low-grade, glossy type black coloration that we have found evidence for in the fossil," she added.

"I think you would notice some differences in the way they moved and that would be the moment at which you realized you were looking at something unlike a modern bird."

While Clarke described the hind limb wing as "kind of perplexing," she said for now paleontologists think the dinosaur probably used it as a signal to others, a display, or maybe even to help it balance upon landing.

The analysis offers a new clue to the vast puzzle of how the flight of birds evolved over time, and suggests that Microraptor did not roam by night as previously suggested because today's nocturnal birds do not show iridescence.

"It's a very clear transitional species and also it has this unique characteristic of the wings that help us understand how flight originated," said co-author Matt Shawkey, associate professor of biology at the University of Akron.

"This study gives us an unprecedented glimpse at what this animal looked like when it was alive," said co-author Mark Norell, chair of the American Museum of Natural History's Division of Paleontology.

"There's been a lot of speculation about how the feathers of Microraptor were oriented and whether they formed airfoils for flight or whether they had to do with sexual display," he added.

"So while we've nailed down what color this animal was, even more importantly, we've determined that Microraptor, like many modern birds, most likely used its ornate feathering to give visual social signals."

The research appears in the journal Science.