Until recently, communication in nocturnal and crepuscular species was thought to follow acoustic and chemical pathways, unlike diurnal birds that use contrasting plumage patches and intricate feather structures to convey visual information. However, several birds that are active in low light have developed very white plumage patches in otherwise unnoticeable feathers.
A new study from Imperial College London uses spectrophotometry, electron microscopy and optical modelling. It explained the mechanisms that produce bright white tail feather tips of the Eurasian woodcock Scolopax rusticola.
The underside of the tail of the Eurasian woodcock, Scolopax rusticola, is covered with patches of white feathers in addition to the main coat of brown spots. This indicates that these spots are only visible when the animal raises its tail or performs courtship flights.
The Eurasian woodcock, Scolopax rusticola, has a brown-spotted primary coat and spots of white feathers on the underside of its tail. This suggests that the animal shows these spots only when raising its tail or participating in courtship flights.
Using specialized microscopy, scientists imaged the feather’s structure, spectrophotometry to measure light reflectance, and models to characterize how photons of light interact with structures within the feather. To their surprise, they found that reflectance measurements showed that the feathers reflected up to 55% of the light – 30% more than any other feather measured.
Based on this, scientists concluded that the mainly brown woodcock uses its bright white tail feathers to communicate in the semi-darkness. The findings suggest much to learn about how birds are most active at night or communicate at dawn and dusk.
Individual feathers are made of a central stem with projections called rami that form the mass of the structure. The rami are held together by round Velcro-like ‘beards’.
The team found that in the woodcock’s white tail feathers, the rami are thickened and flattened, increasing the surface area on which light can bounce and making it less likely that light will pass between the feather barbs without being reflected.
There are two main ways that surfaces are reflective: specular and diffuse. ‘Specular’ reflection is when light bounces off a smooth surface, such as a mirror. ‘Diffuse’ reflection scatters light rays in different directions. The thickened rami were found to consist of a network of keratin nanofibers and scattered air sacs. This creates many interfaces that can scatter light, increasing the diffuse reflectance of the feathers.
The rami and barbules in the white woodcock’s feathers are positioned to create a venetian blind-like effect that further increases surface area by resting at the best angle for light reflection, according to an analysis of the feathers.
Natural History Museum Chief Curator of Birds, Dr. Alex Bond, said: “This research is a brilliant combination of using museum specimens and advanced tools to try to understand this phenomenon. To see if closely related species or species with a similar ecology also had these incredibly white feathers was an important part of the figuring out the story.
Magazine reference:
- Jamie Dunning, Anvay Patil, et al. How Woodcocks Produce the Most Brilliant White Spots in Plumage Among Birds. Royal Society interface. DOI: 10.1098/rsif.2022.0920
