The nocturnal bird with ultrabright feathers

The drab brown coloring of the Eurasian woodcock (Scolopax rusticola) is ideal for providing camouflage during the day and limiting light reflectance at night, when the bird is most active. But when the need arises—most likely, for the purposes of mating, asserting territorial dominance, and other communication—S. rusticola can raise its tail to reveal feather patches that are brilliantly white. A new analysis of those feathers reveals features that collectively reflect broadband visible light more effectively than does the plumage of any other bird.

Jamie Dunning of Imperial College London and his colleagues examined several samples of molted Eurasian woodcock tail feathers at a variety of magnifications. With the naked eye, it’s clear that the white barbs, called rami, that protrude from the central stem on the underside of the tail feather are thicker and flatter than the brown rami on the upper side, enhancing specular reflectivity. Zooming in farther with an optical microscope, the researchers determined that the white rami are oriented at 70° relative to the feather surface and that they overlap each other, like the slats of venetian blinds. Those attributes presumably optimize reflectance and prevent light from passing through the feather.

Higher-magnification views with scanning and transmission electron microscopes revealed the most remarkable features. Viewing cross sections of the rami tips, Dunning’s team observed intricate matrices of keratin fibers. The fibers and the air pockets they surround form photonic cells that effectively scatter light, thus maximizing diffuse reflectivity. The brown rami did not have any such cells.

With the help of all those light-manipulating mechanisms, S. rusticola can maximize reflectance of the available ambient light during its evening ventures. Dunning and colleagues found that the white tail-feather patch reflects more than half the light that strikes it over much of the visible spectrum, peaking at 55% reflectance. To contextualize their reflectance findings, the researchers compared them with analogous measurements of 61 bird species , including nocturnal birds like the common nighthawk. As shown in the graph, the Eurasian woodcock feathers have what the researchers call “the whitest white plumage patch currently known among the birds.” The feathers reflect nearly a third more light than the next most reflective feathers, belonging to the Caspian tern.

Although S. rusticola currently possesses the whitest-known feathers, it may not even reign supreme among the woodcocks. The researchers found white patches with similar visible features to the Eurasian woodcock’s in the tail feathers of the genera’s seven other species. Those plumages have yet to be investigated in detail. (J. Dunning et al., J. R. Soc. Interface 20, 20220920, 2023 .)