Eurasian woodcocks have the brightest white feathers ever measured

The mostly brown Eurasian woodcock uses its brilliant white tail feathers to communicate in semi-darkness, reflecting 30% more light than any other known bird.

These startling findings, by a team led by a scientist from Imperial College London, suggest there’s a lot to be learned about how birds that are most active at night or at dawn and dusk communicate .

The most active birds during the day often have colorful plumages, which they use to communicate information with each other. Birds that are most active at dawn and dusk or at night (“crepuscular”), such as nightjars and woodcocks, tend to have less showy plumage, since while sleeping during the day they must be camouflaged to avoid predators.

Rather than using showy plumages, birds active in low light conditions were thought to instead use sounds or chemicals to communicate. However, many have bright white spots, which could be used in environments with very little natural light for communication if sufficiently reflective.

The woodcock is mostly spotted brown, but it has spots of white feathers on the underside of its tail. This means that it only shows these patches when raising its tail or during courtship flights.

Light microscopy image of the white tail region. Photo by Liliana D’Alba

However, since they are crepuscular and therefore more active in low light, these white spots need to reflect as much light as possible to attract attention. To determine how they might do this, the team studied the white tail feathers of woodcock specimens from a collection in Switzerland.

They used specialized microscopy to image the structure of the feather, spectrophotometry to measure the reflectance of light, and models to characterize how photons of light interact with structures inside the feather. They were surprised to find that the reflectance measurements showed that the feathers reflected up to 55% of the light, which is 30% more light than any other feather measured. The results were published on March 1 in Royal society interface.

Lead researcher Jamie Dunning, from Imperial’s Department of Life Sciences, said: “Bird enthusiasts have long known that woodcocks have these intense white spots, but just how white they are and how they work remained a mystery. From an ecological point of view, the intensity of the reflectance of these feathers makes sense – they must suck in all available light in a very dimly lit environment, under the forest canopy at night.

Individual feathers consist of a central shaft with protuberances called rami forming most of the structure. The branches are held together by round Velcro-like barbs.

The team found that in the woodcock’s white tail feathers, the branches are thickened and flattened, increasing the surface area for light reflection, while reducing the likelihood of light passing between the feather barbs without be thoughtful.

Surfaces are reflective in two main ways. “Specular” reflection occurs when light bounces off a smooth surface, such as a mirror. “Diffused” reflectance scatters light rays in different directions. The thickened twigs were found to consist of a network of keratin nanofibers and scattered air pockets. This creates many interfaces that can scatter light, increasing the diffuse reflectance of feathers.

Feather analysis showed one final trick in the woodcock sleeve: the twigs and barbules of white woodcock feathers are arranged to create a Venetian blind effect that further enhances the surface, sitting at the corner optimal for light reflection.

“This research is a brilliant combination of using museum specimens and cutting-edge tools to try to understand this phenomenon,” said Alex Bond, senior curator of birds at the Natural History Museum. “Being able to see if closely related species or species with similar ecology also had these incredibly white feathers was key to understanding the story.”

Thanks to Imperial College London for providing this news.

Patches in birds’ tail feathers can trick you into identifying them