by Matt Walker
Editor, Earth News
The silver wings of a bone-eating bearded vulture
Some birds have silver wings, created by a previously unknown structure in their feathers, scientists have discovered.
A delicate arrangement of barbules creates a silver sheen upon otherwise dark feathers.
The effect is different to the usual iridescence that adds colour to the plumage of many birds.
Pelicans, ducks, vultures and cranes all possess these silver wings, which might indicate a bird's fitness.
Researchers have published details of this novel plumage in the Journal of Avian Biology.
A group of scientists based in Spain and Canada led by Dr Ismael Galvan of the National Museum of Natural Sciences in Madrid made the discovery when studying the plumage of the bearded vulture (Gypaetus barbatus), also known as the Lammergeier.
"We fell in love with the bearded vulture, a striking, rare bird of the Pyrenees. What struck us was how conspicuous the adult birds were, especially in the sun. The wing, back and tail feathers of these birds had a silvery sheen," says Dr Galvan.
"Later, one of us was handling red-footed boobies and was immediately struck by how much their flight feathers on the wing resembled those of the bearded vulture."
Intrigued, the researchers then searched museum collections of birds held in Madrid, and at the Donana Biological Station in Sevilla, Spain and University of Saskatchewan in Saskatoon, Canada. There, they found examples of a host of other large, open-habitat birds that possessed this trait.
But smaller species such as songbirds didn't have silvery wings.
The sheen of the flight feathers of an adult Egyptian vulture.
The researchers suspected that this sheen may be generated not by pigments, but by the way the feathers of some birds are constructed.
So they examined a series of bearded vulture feathers under light and scanning electron microscopes.
Bird feathers are typically made from a main shaft, called a rachis. From this branch off a series of smaller perpendicular elements called barbs. And from these barbs branch off a series of even smaller elements called barbules, which run almost parallel with the rachis and hook together to give a feather its solidity.
The microscope images revealed that birds with silver wings have barbules that are constructed differently.
Each specialised barbule is flattened, with one side being black and the other translucent. The barbules are longer than usual, and crucially, they twist.
That means that a significant length of translucent barbule is exposed, which creates a silver, white sheen along a whole feather.
A scanning electron microscope shows the structure of a sheen feather of an adult bearded vulture. The barbules twist, displaying the sheen colour of their dorsal surface, and some are broken at the point where they twist (Image: Ismael Galvan)
"This is how the sheen colouration is generated, without the occurrence of coherent scattering like in 'traditional' structural feathers," says Dr Galvan.
They further tested the feathers by scraping them with a pair of scissors, to intentionally damage the specialised barbules. When they did so, the silver sheen disappeared.
"What is really a surprise is how fragile this structure is," explains Dr Galvan.
"One can easily scrape off the colour with a dull blade, and so old feathers on birds are easily recognised from new ones. In some cases the colour shifts from white to black."
They were also surprised to find the structure in the first place.
"Bird colouration has also been intensively and extensively studied for a long time," says Dr Galvan.
"But this structure has been overlooked. This is very odd indeed when you see what a dramatic effect it has on colouration."
The researchers suspect that dark coloured birds evolved this silver sheen as a way to show off, while maintaining the benefits of dark coloured feathers.
Dark feathers contain the pigment melanin, which protects feathers against the damaging effects of abrasion and UV radiation.
"Sheen feathers may represent an alternative way to become more colourful and conspicuous," says Dr Galvan.
"This mechanism can generate brightness in feathers that are in fact dark."