New research led by Imperial College London suggests that Eurasian woodcocks (Scolopax rusticola) have evolved their bright white feather patches for long-range visual communication in dimly lit environments.
The Eurasian woodcock (Scolopax rusticola). Image credit: Serge Santiago.
Diurnal birds (most active 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, as while sleeping during the day they need to be camouflaged to avoid predators.
Rather than using showy plumages, it was thought that birds active during low light conditions instead used sounds or chemicals to communicate.
However, many have bright white patches, which could be used in environments with very little natural light for communication if these are reflective enough.
“Until recently, and when compared with diurnal birds that use contrasting plumage patches and complex feather structures to convey visual information, communication in nocturnal and crepuscular species was considered to follow acoustic and chemical channels,” said Imperial College London researcher Jamie Dunnin and colleagues.
“However, many birds that are active in low-light environments have evolved intensely white plumage patches within otherwise inconspicuous plumages.”
Eurasian woodcocks are primarily mottled brown, but have patches of white feathers on the underside of the tail.
This means the birds only show these patches when raising their tail or during courtship display flights.
However, as they are crepuscular, and so most active during low light, these white patches need to reflect as much light as possible to attract attention.
To investigate how they might do this, the researchers examined the white tail feathers of Eurasian woodcock specimens from a collection in Switzerland.
They used specialised microscopy to image feather structure, spectrophotometry to measure the light reflectance, and models to characterize how photons interact with structures inside the feather.
They were surprised to find the reflectance measurements showed the feathers reflected up to 55% of light — 30% more light than any other measured feather.
“Bird enthusiasts have long known that woodcocks have these intense white patches, but just how white they are and how they function has remained a mystery,” Dr. Dunning said.
“From an ecological perspective the intensity of the reflectance from these feathers makes sense — they need to hoover up all the light available in a very dimly lit environment, under the woodland canopy at night.”
“Individual feathers are made of a central stem with protrusions called rami forming the bulk of the structure. The rami are held together by round Velcro-like barbules.”
The team found that in the woodcock’s white tail feathers the rami are thickened and flattened, which both increases the surface area for light to bounce off, while also making it less likely light will pass between the feather barbs without being reflected.
There are two main ways surfaces are reflective: specular reflection is when light bounces off a smooth surface, like a mirror; diffuse reflectance scatters light rays in different directions.
The thickened rami were found to be made up of a network of keratin nanofibers and scattered air pockets.
This creates lots of interfaces that can scatter light, increasing the feathers’ diffuse reflectance.
Further analysis s showed one final trick up the woodcock’s sleeve: the rami and barbules in the white woodcock feathers are arranged to create a venetian-blind-like effect that further enhances the surface area, by sitting at the optimum angle for light reflectance.
“This research is a brilliant combination of using museum specimens and cutting-edge tools to try and understand this phenomenon,” said Dr. Alex Bond, principal curator of birds at the Natural History Museum in Tring.
“Being able to see whether closely related species or species with similar ecology also had these incredibly white feathers was a key bit of figuring out the story.”
The study is published in the Journal of the Royal Society Interface.
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Jamie Dunning et al. 2023. How woodcocks produce the most brilliant white plumage patches among the birds. J. R. Soc. Interface 20 (200): 20220920; doi: 10.1098/rsif.2022.0920
