Looking out at her quaint Alabama backyard in late January, Charlie Stephenson noticed something unusual. A strange species of unfamiliar yellow bird was pecking at her hanging birdfeeder.

As a seasoned birdwatcher, Stephenson had seen scores of cardinals in the past. But with its mustard-color coat, this flier was different. So, she took a photo with her iPhone and posted it on Facebook.

Stephenson told AL.com she has seen albino and leucistic birds, the latter being animals that are mostly white but can produce some pigment. (Related: "Why Yellow Birds Mysteriously Turn Red.")

But this golden visitor is neither: it's a male northern cardinal with a "one in a million" genetic mutation that made its red feathers yellow, according to Geoffrey Hill, a bird curator at Auburn University in Alabama.

In his 40 years of cardinal birdwatching, Hill has never seen a yellow bird like this in the wild.

The unusually yellow cardinal is not to be confused with the yellow cardinal, an endangered South American species with black and white markings and the occasional green tinge. (Related: "Extremely Rare Albino Orangutan Found in Indonesia.")

Seeking Songbird

After Stephenson posted the photo, her friend and professional photographer Jeremy Black came over to photograph the bird.

"At the time, I had no idea one even existed," Black says. "I kind of thought it was Photoshopped for a second."

On February 17, Black spent five hours scouring Stephenson's backyard with his camera and a pair of binoculars—but after seeing only red cardinals, he withdrew to Stephenson's screened-in porch.

Then, a few hours later, the elusive songbird landed in her neighbor's yard.

"As soon as it landed, I was starstruck," Black says. "It kind of took my breath away a little bit."

Black could only manage two or three shots before a squirrel startled the bird and it flitted away again. He's been trying to get more photos ever since, visiting her yard daily for the last week or so.

Red or Yellow?

Songbirds get their color from yellow, orange, and red pigments called carotenoids found in their food, like sweet potatoes and carrots. Although wild songbirds typically eat yellow-pigmented foods, they can transform that color into warm, red feathers. (Related: "Why These Giraffes Are Completely White.")

But a rare mutation residing in the genes of the cardinal in question might be blocking that color-changing pathway, diluting the bird's red pigment to yellow.

Genetics might not be the only thing to blame for this odd-looking cardinal. The bird's discoloration could also be a sign of illness, Geoff LeBaron, Christmas bird Director at the National Audubon Society, said on the organization's website.

LeBaron points out that the bird's crest and wing feathers are rather worse for wear, a possible indication of a poor diet or stressful environment—factors that may be preventing the bird from boasting its true, vibrant red hue. (Related: "Why This Swedish Moose is Entirely White.")

If the same yellow bird is seen in Alabaster next winter, then it likely has a genetic mutation, he says. DNA analysis could definitively solve this color-confused mystery, though there are no plans to capture the animal. (Related: "How Color-Changing Animals Are Rebelling Against Climate Change.")

"Time will tell with this bird," LeBaron says.