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Many species of songbirds have evolved to have strikingly colorful plumage, aiding them in catching the eye of mates, signaling, and recognizing each other. These eye-catching patterns have long interested scientists, who previously assumed the birds' bright colors arose from only the visible feather layer. However, a new study, in Science Advances, has shown that these colors are getting a boost in brightness from underlying feather layers, adding more complexity than previously assumed.

The researchers examined a range of feathers from tanagers, songbirds with brightly colored plumage caused by both carotenoid pigments and keratin nanostructures within their feathers. The carotenoid pigments鈥攖he same pigments that give carrots and pumpkins their bright colors鈥攚ork by absorbing green and blue wavelengths of light, making them invisible to observers. Meanwhile, only the oranges, yellows and reds get reflected to the observer. The keratin nanostructures, on the other hand, work by scattering the green or blue wavelengths of light back to the observer.

Tanagers, along with many other birds, don't actually have monochromatic feathers. Instead, they are made up of three parts: a soft, downy section near the skin, followed by an achromatic middle section, and then the usually more colorful end section. Because the feathers are layered in an overlapping manner, similar to roof tiles, only the end part is visible. This creates a layered effect, with the more colorful ends, like the outer layer and a middle layer, which is often black or white. Then, the downy layer, which is closer to the body, helps to regulate the bird's temperature.

Through a series of measurements involving hyperspectral imaging, multispectral photography, microspectrophotometry, and optical modeling, the researchers found that the middle achromatic layer helped to make the outer layer colors more vivid, particularly in the male birds.

The study authors explain, "The color of the hidden achromatic feather layer in male tanagers varies with the color of the feathers on top. Carotenoid-pigmented plumage (red, yellow) is paired with a hidden white layer, while structurally colored plumage (blue, violet) is paired with a hidden black layer."

The researchers found that the white layers under carotenoid-pigmented feathers help to increase brightness and saturation by reflecting more of the reds and yellows. Conversely, the black layers under structurally colored feathers increase color saturation and decrease brightness. This makes the blues or greens more vivid because the black helps to absorb the reds and yellows, preventing them from scattering back by the deeper layers of skin.

The color differences between male and female birds are well documented in many species, with the female usually having a duller color. Prior to this study, it was often thought that this dichromatism was due to differences in pigment concentration. However, this study found that this is mainly due to differences in these hidden layers.

Tanagers are part of a larger group of birds called passerines, which have a range of color diversity, many of which have carotenoid pigmentation and/or structural coloration, similar to tanagers. To determine how widespread the helpful achromatic layer was in other songbirds, the researchers surveyed other passerines and discovered that these layers are also found in many other passerine birds with bright plumage.

The authors do note that most of the analysis was done on feathers taken from museum specimens, which may have resulted in some limitations. However, the results of the study offer a new perspective on the complexity of color displays in birds and other animals.

The authors are optimistic about the possibilities that might come with further understanding these color enhancing mechanisms, stating, "Further study of structure-pigment interactions in nature could lead to bioinspired designs in areas as diverse as the development of nontoxic alternatives to common dyes or advances in light harvesting technologies."

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