When a yellow northern cardinal alighted on their bird feeder, David and Tina Gourley of Gravel Switch were puzzled. When a second one joined it, they were shocked.
The first bird — bright yellow, not the typical brilliant red — showed up Jan. 21 and has come back frequently, Tina Gourley said. The next weekend, the second one arrived.
"It looked like a cardinal," she said, "but we'd never seen anything like it."
Apparently not many other people, including bird experts, have seen a yellow Northern cardinal, much less two.
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The first bird stayed at the Gourleys' feeder long enough for the Boyle County couple — who enjoy feeding birds but don't describe themselves as birders — to get a few photos of it through a window.
They sent the pictures to the Kentucky Department of Fish and Wildlife, which forwarded them to the Kentucky Ornithological Society.
Society member Brainard Palmer-Ball, a retired zoologist with Kentucky Nature Preserves, contacted Auburn University professor Geoff Hill, who had co-authored a research paper on the coloration phenomenon in 2003. Hill confirmed that a genetic mutation affected the color of the birds' plumage, Palmer-Ball said.
It is evidently a rare mutation.
The first documented yellow Northern cardinal was "collected" in 1989 by the Museum of Natural Science at Louisiana State University, according to Hill's research paper.
Typical cardinals get their rich red feathers by metabolizing their food into a certain pigment, Palmer-Ball said. With the yellow Northern cardinals — not to be confused with a separate South American species known simply as a yellow cardinal — a mutation causes the metabolic process to create a different type of pigment.
He also said that Hill's research showed that, as with albino animals, there is probably something associated with the mutation that affects the birds' overall health and fitness. Because of that, Palmer-Ball said, the birds are less likely to live very long.
He also said they're less likely to reproduce, but that it's possible that one of the yellow cardinals is an offspring of the other. The two also could be siblings.
Palmer-Ball said the most common color mutation that he has seen in birds is albinism, in which a bird has a few white feathers, a patch of white or an overall paleness. He remembers seeing a pink cardinal, the result of what's called dilute albinism.
Tina Gourley said the other birds that come to their feeder don't seem to know what to make of the yellow cardinals.
"It seems like when they come to the feeder, the other birds clear out and let them have it," she said. They've seen only one bold chickadee come to the feeder when the cardinals are there.