Feathers found in Burmese amber dating back 100 million years are so exquisitely preserved that palaeontologists have been able to make a detailed study of their structure - and they're like nothing seen in living birds today.

In fact, they may have served as a type of decoy, falling away in a predator's grasp, much like a lizard drops its tail to make its escape.

The feathers, found in 31 pieces of Myanmar amber dating back to the Cretaceous (commonly known as Burmese amber) were analysed by a team led by palaeontologist Lida Xing of the China University of Geosciences in Beijing.

You may remember Xing from such Burmese amber smash hits as 100 Million Year-Old Bird Trapped in Amber, Unlucky Frogs Trapped in Amber are the Oldest Ever Found, and of course the absolutely epic A Feathered Dinosaur Tail Has Been Found Preserved in Amber.

These feathers now join this list of special finds. They're called tail streamers, and they're long feathers that extend from the tails of these ancient birds - sometimes even longer than the birds themselves.

Because modern birds also often have very long tail feathers for ornamental and mating purposes, it was thought that this is why Cretaceous birds had them, too.

But, although we've known about Cretaceous bird tail feathers for decades, most fossil specimens recovered have been squished flat, which makes a more detailed study of their purpose a little tricky.

The amber specimens - most of which show that the feathers occurred in pairs - are beautifully preserved in all three dimensions. So the team has been able to discern their strange morphology, and understand a little bit about how they might have been used by the birds.

"The way we interpreted these feathers from compression fossils was basically completely, entirely wrong. Looking at them in three dimensions preserved in amber, I was astonished," palaeontologist Jingmai O'Connor of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing told Science.

"They are the weirdest feathers I have ever seen."

They are dominated by the rachis, or central shaft of the feather (hence the other name they are known by, rachis dominated feathers, or RDFs). But, as the researchers have now ascertained, that rachis is quite different from the closed cylinder seen in modern birds.

Rather, it is open on the underside - like a C- or U-shape - with fewer barbs on either side than modern feathers. The rachis could also be incredibly thin - less than 3 micrometres in some cases (a human blood cell is 7 micrometres in diameter on average). Yet they still would have stuck out, straight and rigid.

The thinness and shape of the rachis leads the researchers to believe that the feathers would have had a lower energy cost to grow - a desirable trait if the feathers are disposable, as clues indicate.

For instance, some feather patterns surrounding the RDFs indicate that the feather struck the oozing sap with some force, while others were found without a sign of a dead bird nearby. According to the researchers, both of these features suggest the feathers were easy to remove.

They also weren't as colourful as you'd expect from a sexy tail feather.

"The apparent ease of removal and muted colours observed in amber RDFs may indicate a sacrificial role in defence, as well as usefulness in visual signalling," the researchers wrote in their paper.

"The reduced amount of material involved in building an elongate RDF with an open and thin-walled rachis may have helped to reduce the energetic costs of producing feathers that were in many cases as long as the total body length of their bearers."

However, the strange shape of the rachis seen in these feathers raises more questions - namely, whether the RDFs evolved from normal feathers, or whether they followed a different evolutionary pathway.

This question will, however, require the study of a larger number of RDF amber samples of exceptionally high quality to answer. Fingers crossed scientists can get their hands on some soon.

The team's research has been published in the Journal of Palaeogeography.