In late January, something odd appeared on Charlie Stephenson’s bird feeder in Alabaster, Alabama. It looked just like a cardinal—the bright crimson bird common across the eastern United States—except this one was the color of mustard. As Dennis Pillion at AL.com reports, Stephenson posted iPhone images of the bird on Facebook and biologists soon confirmed that the yellow bird is indeed a male Northern Cardinal with a very rare color variation.

Pillion reports that the bird has continued to visit Stephenson’s feeders at least once a day in the past few weeks, though she isn’t willing to give out her address for fear of being mobbed by bird enthusiasts.

So far, the best images of the bird have come from Stephenson’s friend, professional photographer Jeremy Black, who staked out her backyard last week. “As soon as I saw it on her social media, I was kind of curious and I wanted to go explore and see if I could find it,” Black tells Pillion. “I finally saw it after about five hours.”

"As soon as it landed, I was starstruck," he tells Elaina Zachos at National Geographic. "It kind of took my breath away a little bit."

So why is the normally brilliant red cardinal bright yellow? Researchers aren’t sure exactly, but Purbita Saha at Audubon.com writes that there are a couple of possibilities. One is xanthochroism, a genetic mutation found in insects and some birds which causes red pigments to be replaced by yellow. For some, this is a matter of differences in absorption of food pigments, for others it's reactions within the bird that convert one colored pigment to another. It can also be some combination of both mechanisms.

"Songbirds like cardinals almost never consume red pigments; rather they consume abundant yellow pigments," Hill tells Pillion. "So, to be red, cardinals have to biochemically convert yellow pigments to red."

It’s also possible that the bird is sick, Geoff LeBaron, Audubon’s Christmas Bird Count director tells Saha. He points out that the bird's feathers and crest look frayed, an indicator that the cardinal might be having a hard winter, suffering from environmental stress or a poor diet. That might inhibit the bird’s ability to produce its red color. If that’s the case, LeBaron says, it’s likely that once the bird’s health improves it will lose its yellow feathers during its next molt and turn red. If it hangs around and is still yellow next winter, he says that's a sign that its color likely comes from a mutation.

Hill hopes to get DNA from the bird to find out for sure. In 2016 he was part of a team that first identified the gene, CYP2J19, that is responsible for cardinals' and other birds' red coloring. As Erin Edgemon at AL.com reports, researchers are just beginning to understand the importance of red pigments in birds. Studies have shown that for birds, redder is better: Female birds strongly prefer the brightest red mates. Hill says that’s likely because the enzyme that converts yellow pigments into red is associated with the mitochondria, the energy producing organelles in cells. This suggests that the redder a bird, the better it is at producing energy.

There have been other recent breakthroughs in understanding just how birds produce their colors. Besides discovering CYP2J19, researchers have also learned details of how birds produce structural color. Instead of producing color via a pigment, structural color is created when light is reflected and absorbed by microscopic structures on a bird's feathers. Last month, researchers found that structures on the feathers of some species of birds of paradise can absorb 99.95 percent of light, creating an intense black. Blue feathers also get their color from microstructures, since digestion destroys blue pigments in birds. Such tricks of light are also used by intensely blue animals such as peacocks, Morpho butterflies and the awesome blue tarantulas discovered in Guyana last year.

This isn’t the only strange cardinal found in recent years. A 2014 study detailed a cardinal that had female plumage on one side of its body and male plumage on the other. The gynandromorph, as it is known, has a mixture of male and female genes and expresses traits from both sexes. So keep your eyes to the sky, perhaps you too will spot a bird of a different feather.