A 121-million-year-old baby arboreal bird, fossilised while still curled in its egg, has been found in China, Science magazine reports this week.
Most modern arboreal birds are born completely helpless
The fossil is thought to be the most ancient unborn bird ever discovered.
It has piqued researchers' interest because it had feathers, whereas many modern flying birds are naked and helpless when they first hatch.
The authors say this supports the view that birds developed the strategy of hatching featherless later in history.
"This fossil is interesting because its preservation is so exceptionally fine, that even the soft tissues like feathers have been preserved," commented Dr Angela Milner of London's Natural History Museum.
"For an embryo that is still inside the egg, it is surprising how advanced the feathers were."
The researchers know the bird, found in northeast China, was an embryo because the fossil is tucked up in very characteristic way for an unhatched chick.
"The tucked posture of the fossil is consistent with a late-stage embryo rather than with a hatchling, in which case the head would have raised beyond the vicinity of the feet," said authors Zhonghe Zhou and Fucheng Zhang from the Chinese Academy of Sciences in Beijing, China.
But apart from the chick's posture, it was not very babyish at all.
"The interesting thing about this bird is that for something that has not yet hatched, it is almost fully formed," said Dr Milner.
"All its bones are formed and its feathers are very well developed."
This maturity means the bird must have been "prococial".
Prococial birds - like chickens, ducks and ostriches - produce young which are immediately competent: they have downy feathers, can run about and feed themselves almost as soon as they hatch.
"Altricial" birds on the other hand, like all songbirds, are born completely helpless, naked and blind. They require devoted care from their parents in order to survive.
Most modern arboreal (tree dwelling) birds are altricial so they can grow to almost full-size in a protected environment, like a nest, before they must attempt the risky business of flying.
"If you are an altricial bird you are kept warm and well fed," explained Dr Milner.
"You are able to grow big and strong while you are in the nest and you are ready to fly at full size when you leave the nest."
The fossil is thought to be the oldest known unborn bird ever discovered
The fact that this bird - which lived in the Lower Cretaceous Period - was prococial could suggest it did not have the same luxury as its modern day counterparts.
It may have been forced to make its own way in the world much sooner.
"The fact that its feathers were so well developed could mean that these things could fly quite soon after they hatched," said Dr Milner.
This fossil supports the notion that Earth's first birds had not yet developed the altricial strategy, with all the intensive parental care that it entails.
"It is generally believed that precociality is ancient and altriciality is derived," said Dr Zhou and Dr Zhang in their paper.
In a separate development, a new fossil bird discovered in China may shed light on a controversial theory of the origin of flight.
Some scientists think birds went through a "four-winged" stage in their evolution before the tail evolved its current aerodynamic shape, freeing the legs from flight duties.
In the journal Nature, Fucheng Zhang and Zhonghe Zhou describe a fossilised bird from Early Cretaceous times which had very long feathers on its legs.
The bird, which belongs to a group of early birds known as enantiornithines, is between 145 million and 125 million years old.
The feathers could be "remnants of earlier long, aerodynamic leg feathers, in keeping with the hypothesis that birds went through a four-winged stage during the evolution of flight", say the researchers.
Palaeontologists have uncovered evidence of a four-winged feathered dinosaur called Microraptor. The squirrel-sized creature used the long feathers on all four of its limbs to glide or parachute from tree to tree.