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These pictures were taken by David and Marilyn Gourley who feed birds in their backyard but are not birders. A posting from the Kentucky Ornithological Society describes the pictures taken in Boyle County, KY in January.
Geoff Hill of Auburn University has published a paper, available in PDF, based on a yellow Northern Cardinal which is in the collection at LSU. Their specimen "lacked all four of the typical red pigments, and produced alternate unique pigments. This mutation apparently occurs randomly and is extremely rare."
Hill says, "The mutation is associated with fitness costs so it doesn't spread in areas." You might also wonder if the cardinal is able to find a mate, with all the females saying "are you going to wear that to the feeder tonight?"
Sent to me by George Deatz of Friends of the Garden.