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How do birds get their colors? A guide to your vibrantly hued, winged friends

Birds boast some of the most vibrant colors among terrestrial animals. The tropics are often renowned for the brightest birds, but U.S. birds also display a wide array of striking hues.

But just how colorful are our avian friends?

ӣƵ analyzed over 500 photographs of bird species provided by the at the , whose extensive media collection helps researchers identify and study color abnormalities and rare bird colors. The species were chosen based on data from Project FeederWatch, a citizen science program run by the Cornell Lab, which allows people to submit sightings of birds they've seen locally.

Using a machine learning algorithm, we determined the dominant color of each bird photo.

Let's take a look at the American kestrel, one of the smallest and most colorful falcons in the U.S. It can be spotted all over the country, soaring over open expanses while looking for insects to eat. Running the photo through a machine learning algorithm resulted in this color palette for the bird.

The machine learning model determined that the dominant color for this bird is dark gray, which means this color group contained the most pixels. We ran this same analysis for all 559 bird species, resulting in a rainbow of colors representing U.S. birds.

It's important to mention that the dominant color we got depends on the lighting of the photo, the time of year it was taken, the sex of the bird and its age.

Avian colors do more than just captivate human spectators. They also serve utilitarian purposes, indicating overall environmental quality and playing important roles in a bird's survival and courtship behavior.

“Birds are sentinels of the environment,” said , the associate curator of ornithology at the Natural History Museums of Los Angeles County in California.

"The artist in me is what really fell in love with bird colors," Shultz said, "but it was the biologist in me that discovered why they're so important and that they're actually worth studying."

How do birds get their colors?

Understanding bird coloration combines biology and physics. There are two primary ways that birds get their color: pigmentation and the physical structure of the feather – sometimes a combination of both.

Pigments are the chemical substances in animal and plant cells that impart color, absorb and reflect certain wavelengths of light.

Birds acquire some pigments, such as carotenoids, by eating fruits, seeds, or insects. Carotenoids produce the bright reds, yellows, and oranges seen in birds, and they are the same pigments that give carrots and pumpkins their signature color.

The other way birds acquire their appearance is through structural colors, which result from the interaction of light with the microscopic structures in feathers. Tiny air bubbles within the feather refract and scatter light, creating vibrant colors for us to perceive.

Picture the surface of a soap bubble or a rainbow-like oil slick. These phenomena, along with morpho butterflies’ brilliant blue wings, are examples of structural colors at play.

The , a classic red bird, has pigment in its feathers that absorbs all but the red wavelengths, which are then reflected to us.

Carotenoid-based colors are often used as an indicator of a male bird’s quality, Shultz said. Brighter colors are frequently associated with , stronger immune systems and higher reproductive success.

This concept, known as “redder is better,” reflects the idea that brighter birds make for better mates, said , an evolutionary ornithologist who teaches at Yale University. However, Prum disagrees and says the concept is a problematic way of "taking beauty in the world and turning it into utility.”

While several North American birds exhibit apparent green plumage, turacos, native to sub-Saharan Africa, stand out as the only birds that are . Unlike other species, turacos owe their color to a copper-based pigment called turacoverdin.

The common grackle and many shimmering hummingbirds display iridescence like the way a prism splits light into a rainbow. Their iridescent feathers change color depending on the viewing angle, adding to the allure.

Colors of a bird

While bird species dazzle with myriad colors, a single bird usually has several colors itself, which our color palettes above don’t capture.

The colors that birds have boil down to evolutionary trade-offs and selective pressures, Prum said.

The strongest pressure is what drives a bird’s feathers to be a certain way, Shultz said, whether it’s to impress a mate, blend into the environment, or stay cool in a hot place.

While bright colors can be great for social and sexual signaling, they’re not as helpful for avoiding predators.

Bright pops of color tend to be signaling colors used for social interactions. It can be a way of saying, “Join my flock. We're members of the same flock,” Prum said.

For this reason, these vibrant spots often appear on a bird’s crown, throat, or breast – areas that face other birds, Shultz said. A bird’s back is typically darker and more cryptic, aiding in camouflage. This adaptation, known as , helps the bird blend in and avoid drawing attention to itself.

Male ruby-crowned kinglets, for example, can flash a scarlet-red crown when excited or keep it concealed when needed.

Feathers containing melanin are stronger, Shultz said, which is why birds often have dark wing feathers to aid them in flight.

Do colors change?

Even within the same species, color can vary by age, season and sex.

Birds undergo molting, the process of shedding old feathers and growing new ones, which can result in color transformations. Most birds experience an annual molt, typically after the breeding season, Shultz said, but species like American goldfinches replace their feathers twice a year.

“If you're looking at birds in certain times of year when they're molting, they might look a little bit scraggly,” Shultz said. “That's just because they're growing new feathers.”

Molting helps birds maintain optimal feather condition for flight and allows them to replace worn-out feathers and remove parasites, Prum said.

Young birds might also look different than their older counterparts. That’s because it takes time for birds to signal sexual maturity. Most songbirds reach this state after one year, but bald eagles can take as long as to attain full adult plumage.

Males and females can have varying appearances –a phenomenon called sexual dimorphism. Females tend to be drabber because they have different ecological and social behavioral needs, although sometimes, the two sexes look identical, as seen in blue jays.

How do birds see colors?

As colorful as birds are to the human eye, we’re actually “colorblind with respect to birds,” Prum said. That’s because birds see an even wider gamut of colors than humans can.

“Birds are living in a much more vibrant and colorful world than we are,” Shultz said.

Humans have three types of cones in our eyes for red, green and blue light. Birds have an added type of cone that allows them to see ultraviolet light.

In addition to seeing a broader spectrum of colors, birds can also detect finer differences between them. That means birds can distinguish subtle nuances in shades of green that are not perceptible to us.

“Everybody always asks me, ‘Could you make bird vision goggles?’” Shultz said. “And really, we can't because our brains are just not set up to neurologically process that type of information.”

Curious to learn more about the birds you've seen in this story? Click on a color to find out.

Contributing: Javier Zarracina

Sources: U.S. Fish and Wildlife Service National Digital Library, Getty Images.

Methodology: Photos were manually selected to find images of birds in a similar pose, with good lighting and for overall quality. The background was removed from all photos and each photo was run through the K-means algorithm to get the dominant color. K-means works by dividing the colors of a bird into distinct groups, or 'clusters' to find the most representative colors. Each pixel's color is then assigned to the nearest cluster center. The cluster with the most pixels becomes the dominant color.

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