Scientists in Myanmar have discovered what they describe as the "most complete" fossil of a baby bird ever recovered from the Cretaceous period, trapped in 99-million-year-old amber.

The hatchling, complete with feathers, claws, skin and soft tissue around the eyelid and external opening of the ear, is believed to be a species from a group of birds called enantiornitheans that went extinct about 65 million years ago.

Almost all enantiornitheans had teeth, and each finger within the wing contained a claw.

In a paper published in Gondwana Research the scientists said the find offered new insight into "the most species-rich clade of pre-modern birds" to have ever existed.

"The new amber specimen yields the most complete view of hatchling plumage and integument yet to be recovered from the Cretaceous," the paper said.

Based on the presence of "flight feathers" ranging in colour from white to brown and dark grey, the bird is thought to have been capable of flight at or very soon after birth, but became trapped in amber at just a few days old.

"The plumage preserves an unusual combination of [developed] and [undeveloped] features unlike any living hatchling bird," the paper said.

Researchers believed the early ability to fly would have helped the birds flee predators, but the high number of young enantiornitheans in the fossil record suggested their independence came at a cost.

Myanmar valley a hotbed of Cretaceous fossils

The presence of "flight feathers" suggests the bird could fly when hatched. ( Chinese Academy of Sciences: Ming Bai )

The Hukawng Valley in northern Myanmar where the specimen was found is known for its rich amber deposits and is believed to contain the largest variety of animal and plant fossils from the Cretaceous period, 145.5 to 65.5 million years ago.

In 2016, a feathered dinosaur tail was discovered in a piece of amber that had been purchased in 2014 from a market in Myanmar, where it was being sold as jewellery.

"[I thought we had] just a pair of feet and some feathers before it underwent CT imaging. It was a big, big, big surprise after that," research team co-leader Lida Xing said of that discovery.

Dr Xing is one of five scientists credited with discovering the dinosaur tail in 2016, involved with the latest discovery of the enantiornithean fossil.