Unique Feather Structure Reveals Why "World's Most Dangerous Bird" Is So Shiny

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Rachael Funnell 13 May 2020, 19:00

If you type “world’s most dangerous bird” into google, a big, blue, dinosaur-footed cassowary is what greets you. These ancient-looking animals, nicknamed "murderbirds," diverged from chickens approximately 100 million years ago. Beyond their enormous size and malevolent appearance, they sport fabulously shiny feathers – though we’d recommend not getting close enough IRL to appreciate them. A new paper published in the journal Science Advances has discovered, for the first time, at a molecular level what makes these fabulous feathers so, well, fabulous, whilst also uncovering details as to the appearance of fossil feathered specimens.

In mammals, the color of fur or skin mostly comes from pigments such as melanin, which is what gives human skin a darker color either in response to our genes or exposure to the Sun. For birds, it’s slightly different as some of the color in feathers is due to the physical makeup of their feathers. Melanosomes in the feathers influence their pigment, depending on how these melanosomes interact with light. Different shapes or configurations of melanosomes produce different colors or effects such as matte or sheen feathers.

While the feather structure and color of neognaths (one of the two groups of extant birds that includes songbirds) have been identified, that of paleognaths, which contains several species of flightless birds, was less well understood. It’s now been discovered that paleognaths such as the cassowary also have structural colors in their feathers, owing to the unusual physical features of their feathers.

Microscopic detail of a cassowary feather, showing the exposed rachises that allow for iridescence. (c) Chad Eliason

Birds’ feathers are not dissimilar to the structure of a tree, having a long trunk running down the middle (known as the rachis) with branches called barbs. The barbs are coated in tiny structures called barbules and the shape of these is what defines if a feather is glossy. However, the researchers found that for cassowary feathers it wasn’t the barbules that influenced the color but instead the rachis –  the central “trunk” of the feather. The rachis in cassowary feathers gets more exposure to light than the barbules as the barbules are quite sparse and fluffy, meaning the rachis is more exposed.

A comparison of the feather structures and iridescent colors of different kinds of birds. Chad Eliason

 As well as discovering the root of cassowary feather coloration, the researchers also took a look back at one of the cassowaries’ cousins that lived 52 million years ago. The long-extinct bird, known as Calxavis grandei, lived in what we now call Wyoming and left behind some very well-preserved specimens, including imprints of the birds’ feathers.

"You can look at a fossil slab and see an outline of where their feathers were, because you kind of see the black stain of melanin that's left over, even after 50 million years or so," said Chad Eliason of the Field Museum and the paper's first author in a statement. "We peeled off little flakes of the fossil from the dark spots of melanin, and then we used scanning electron microscopes to look for remnants of preserved melanosomes."

(Left) Shiny feather structures in a southern cassowary. Chad Eliason. (Right) Color reconstruction of extinct cassowary relative, Calciavis grandei, showing iridescent plumage. Velizar Simeonovski

Microscopic investigation of these feathers revealed the shape of the pigment-producing melanosomes in the feathers’ barbules, finding they were long, skinny, and green bean-shaped, which in modern birds is associated with iridescence. The finding, combined with the new understanding of cassowary feathers, constitutes the first evidence of structural color in paleognath feathers

"It gives us a glimpse into the time when dinosaurs were going extinct and the birds were rising," says Eliason. "Studying these paleognaths gives us a better understanding of what was happening there, because you can't just study neognaths; you need to study both sister clades to understand their ancestors."

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