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When Belgian wildlife photographer Yves Adams stepped off his inflatable Zodiac boat and onto the island of  in the southern Atlantic Ocean, the beach was crammed full of animals. But as Adams scanned the bustling crowd of King Penguins, elephant seals, and Antarctic fur seals, he spotted something bizarre in the distance. 

“Suddenly, we saw this group of King Penguins swimming on the coast,” Adams says. “And then there was this one particular bird that took our attention—a pale bird.” At first, he wasn’t sure what he was looking at. The lifelong birder was leading a photography expedition, and he had studied the island’s species before arriving. What the group saw didn’t look like anything they had expected. “I was a little bit in shock,” he says. “I thought I must have skipped some page in the booklet.” 

A peek through his binoculars confirmed that the bird was a King Penguin, but instead of sporting the species’ typical black, white, and yellow plumage, this one was all white with more yellow highlights than normal. When the bird began swimming closer to Adams and others, he couldn’t believe his luck. “I grabbed my camera, and I started shooting,” he says. “It was spectacular.”

The whole encounter lasted less than a minute before the bird hopped on shore and blended into the colony of thousands of King Penguins. After the sighting, Adams says he couldn't stop thinking about what he witnessed. To his knowledge, no one had ever seen a penguin quite like this one. And now that his photos have gone viral in recent weeks, experts are equally perplexed.

Of the many genetic quirks that can cause abnormal colors, the most well-known is , the inability to synthesize any melanin. Albino birds usually have an all-white appearance, with pink eyes, beak, skin, and feet. But Allison Schultz, curator of ornithology at the Natural History Museum of Los Angeles, doesn't think the penguin spotted by Adams is albino.

Usually, a bird’s yellow, orange, and red plumage is produced by carotenoids, explains Schultz. But penguins have their own unique pigment called , so they could lack all melanin and still retain their sunny hues. Schultz doesn't think that's what going on here, though. She points to color in the bird's eyes and light rufous tones on some feathers as evidence of melanin. Instead, she thinks the bird has . “Albino means that all melanin production is gone, whereas leucism means that there is still some melanin production,” she says. 

Sometimes leucistic birds appear entirely white, while others are missing tiny patches of pigment. In this penguin’s case, it’s missing all black and brown pigment, revealing yellow plumage where it is normally concealed by darker feathers, such as across the neck and on the back of the bird's head. Because there is some presence of color on this bird, Schultz suspects part of the penguin's melanin pathway is simply broken, preventing the melanin from being deposited where it should be and rendering typically dark feature white.

Hein van Grouw, an ornithologist at The Natural History Museum at Tring in the United Kingdom, agrees there is a mutation affecting the bird’s melanin, but he would not say the bird is leucistic. Because the penguin still has a tinge of rusty color on its back, Grouw thinks the bird’s melanin wasn’t fully synthesized —dubbing the bird “” Grouw says that the term ino is more commonly used in Europe to describe melanin that  into its usual dark color, while in the U.S., leucism is used more broadly to describe a range of pigment variations in a bird’s feathers or skin.

So far, the opinions of scientists elsewhere also lean toward leucism, with there being similar disagreements about the type and terminology.

No matter the label, Grouw and Schultz agree that the bird’s appearance is most likely the product of a broken melanin pathway, not albinism. But to know for certain exactly what’s happening with this special King Penguin, Schultz and Grouw say they would have to find and do genetic tests on the bird, which would be like finding a needle in a haystack.

Because this type of genetic mutation is recessive, Grouw says he suspects this penguin is a , and appears to be a young adult. If she mates with another King Penguin who carries this same gene and they have female chicks, they would share her striking all-yellow plumage.

But surviving long enough to find a mate could be a challenge for this penguin. King Penguins’ dark backs help conceal them from predators above, and their white bellies camouflage them from those swimming below. While her bright yellow feathers could make her an easy-to-spot meal, they might also dazzle a mate, says Schultz. “It could really go either way for this penguin.”

As for Adams, he feels fortunate to have seen and been able to photograph such a rare penguin. He has also been delighted by the response to his photos, which he did not expect. "At the time itself, I was not thinking, okay, this is gonna be a picture that is going to go viral," he says, following with a laugh, "maybe it's a little bit more special than I thought."