You don't have to be an ornithologist to know that red northern cardinals are male and brownish-gray ones are female.

But what about a Cardinalis cardinalis that sports red feathers on one side of its body and brownish-gray feathers on the other? Why, that cardinal is half-male and half-female, of course--and just such a rare bird has been observed in northwestern Illinois.

An example of a phenomenon biologists call bilateral gynandromorphism, the bird was observed for more than 40 days between Dec. 2008 and March 2010--and it certainly caught the attention of the scientists who spotted it.

"It was amazing when the bird was viewed from one side it appeared as a normal male and from the opposite side it appeared as a normal female," Dr. Brian D. Peer, a professor of biology at Western Illinois University in Macomb and one of the scientists, told The Huffington Post in an email. "It wasn't until you could see both halves of the bird did you realize it was a truly unique individual."

If the bird looked weird, it also exhibited weird behavior. As Peer and his collaborator wrote in a paper published in the Dec. 2014 edition of the Wilson Journal of Ornithology, it was never heard vocalizing or seen to pair up with another cardinal.

"It never acted like a typical male or typical female cardinal," Peer said in the email.

The bird's bizarre plumage arose because its sex chromosomes didn't segregate properly after fertilization, according to New Scientist.

The researchers tried to capture the cardinal to take DNA samples but were unable to do so, Peer said in the email. However, research on chickens has shown that gynandromorphs tend to have mostly male cells in the half of the body with male plumage and mostly female cells in the half with female plumage.

Bilateral gynandromorphism has also been observed in butterflies, crustaceans, and other birds, according to Science magazine.

Peer said there has been at least one other published account of a bilateral gynandromorph cardinal but its plumage was reversed--male on the right side and female on the left.