An international team of researchers found that woodcock’s white spots reflected up to 55 percent of light, making their feathers about 30 percent more reflective than any other bird previously measured.
Woodcocks are mottled brown birds with white spots on the underside of their tail feathers.
Around spring in Europe and Asia, males use these bright spots to breed display flights to catch the eyes of females on the ground, and females fan out their tail feathers to attract males flying overhead.
This mating ritual takes place at dusk or dawn, when the woodcock is most active.
Most other times the white spots are hidden under mottled brown feathers. These feathers blend in with the leaf litter so the woodcock can search around for earthworms and insects without attracting predators.
“Bird enthusiasts have long known that woodpeckers have these intense white spots,” said lead researcher and ornithologist at Imperial College London, Jamie Dunning.
“From an ecological perspective, the intensity of the reflectance of these feathers makes sense – they must pick up all the available light in a very dimly lit environment, under the forest canopy at night.”
Researchers retrieved tail feathers from a collection in Switzerland to find out how bright the woodcock’s feathers were. They used electron microscopy to analyze the structure, spectrophotometry to measure light reflectance, and optical modeling to track how light interacted with structures inside the feather.
After comparing the results with those of other species, the researchers reported that the woodcock had “the whitest white plumage patch currently known among birds”.
The woodcock’s white spots reflected up to 55 percent of the light. This was 31 percent brighter than the nearest comparator, the Caspian tern (Hydroprogne caspia), which reflected up to 38 percent of the light.
Macro and microscopic structures were responsible for this brilliant, reflective effect.
Within a single feather, the branches, called rami, were flattened and overlapped like blinds.
This increased the surface area available for reflection and prevented light from traveling through the cracks between branches.
The thickened branches of feathers (rami) were better at scattering light in many directions because they consisted of a network of keratin nanofibers and air pockets.
All of this came together to produce the whitest white feathers that woodpeckers use to communicate in dimly lit environments.
This paper was published in Royal Society Interface.