For the yellow-shafted northern flicker, “you are what you eat” has proven freakishly true.

These eastern North American woodpeckers get their name from a thin vein of yellow that runs through the center of their dark feathers.

In the past few decades, ornithologists have noticed some the birds' yellow feathers have mysteriously turned red. (See National Geographic's backyard bird identifier.)

At first researchers thought the birds were somehow interbreeding with the red-shafted northern flicker, which is lives in the West. Yet some of the oddly red birds were thousands of miles away from their western cousins.

The answer, it turned out, had to do with their diets: The birds were eating red berries that turn their feathers crimson, according to new research published in The Auk. Vibrant yellow, orange, and red hues common in bird plumage actually come from pigments in the food they eat.

View Images Normally yellow feathers in some northern yellow-shafted flickers are turning red, as seen above. Photograph by C. Hansen

“This answers a really important question,” says study leader Jocelyn Hudon, an ornithologist at the Royal Alberta Museum in Canada. “Colors are an important signal in birds, so a change in color can have huge impacts," such as finding a mate, she adds.

Bird of a Different Feather

Hudon's first clue came in previous studies of the cedar waxwing.

In the 1960s, biologists noticed the yellow tips of some birds' feathers were turning orange.

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The cedar waxwing didn’t have any close relatives with red feathers, so the scientists soon turned to the bird’s food.

Waxwings love berries—so much so that birds have died of intoxication after eating too many overripe, fermented berries. But they've also co-evolved with many of these plants, so it didn’t make sense that their feathers would abruptly change color.

Further studies revealed the birds were eating invasive honeysuckle berries, which were making their feathers red.

In the late 1800s, horticulturalists imported honeysuckle bushes from Europe and Asia both as landscaping and to provide bird habitat and food.

View Images The red-shafted northern flicker lives in western North America, far from where the strangely red northern yellow-shafted flickers live. Photograph by All Canada Photos, Alamy

The shrubs, however, rapidly spread over the continent, becoming an invasive species.

Red-Feathered Mystery Solved

Despite the waxwing findings, bird experts puzzling over the red northern flickers still suspected interbreeding was the cause.

So, using museum samples, Hudon and colleagues compared pigments of various flicker feathers with those of honeysuckle berries.

Although the red pigments from the normally red-shafted flickers and the honeysuckle berries looked similar, they absorbed different wavelengths of light. (Take National Geographic's bird quiz.)

This, and chemical analysis of both pigments, showed that the red hue in the flickers' color-changing feathers came from honeysuckle berries.

“It’s a good piece of detective work that explains a known phenomenon and says something about how birds acquire the pigments in their feathers,” says William Moore, an ornithologist at Wayne State University who wasn't involved in the study.

No one knows if this could affect the northern flicker’s ability to select a good mate, or how many other bird species may be experiencing similar phenomena.

It’s another example, he adds, of the ripple effects of invasive species.