It’s a xanthowhat?

Bilateral gynandromorphism. Great term, don’t you think? It refers to the situation in which an animal shows male characteristics on one side of its body and female characteristics on the other.

Although rare, it is a condition that has been noted most often in insects and marine invertebrates, but occasionally in birds. Photos and a story about a bilateral gynandromorph northern cardinal that was studied by researchers made the rounds on the Web and social media not too long ago.

This is one of the rarest of a number of interesting and unusual plumage aberrations that is possible to discover when we look closely at the birds in our backyards and neighborhoods.

Recently, our friend and birder colleague Bill Sheehan, whose stomping grounds are up in far northern Maine, posted some photos of an amazingly bright yellow evening grosbeak that, as he said, looked like “a great big canary.” As Bill reported from his research into the distinctive bird, there are a number of wonderful, tongue-twisting technical terms that may apply to a bird like xanthochromism and even better, non-melanic schizochroism — try throwing those into the conversation at your next cocktail party!

Probably the most widely recognized and most often seen abnormal plumage variation in birds is some form of what we most commonly call albinism, when some or all of a bird is all white. Not surprisingly, even this condition gets complicated quickly because scientists typically say that a true albino is an individual that is lacking all normal pigments so that along with being all white, its eyes are pink.

Most of the birds we see, though, either have patches of white in the plumage or are missing only some pigments. The former case is often commonly referred to as “partial albinism” and the latter case as “leucism.”

Some experts would say that both cases are really properly considered leucism. In any case, if you watch the birds at your feeder closely you’ll almost certainly eventually see a bird with white feathers where they should be black or some other color. We have seen white-tailed dark-eyed juncos and black-capped chickadees, for example, a number of times. A leucistic bohemian waxwing that was recently photographed in Jay, Maine, was recently reported on the Maine Birds listerve.

A rarer plumage aberration is that of melanism, when black pigment replaces other pigments. In some bird and mammal species there are regional populations or portions of a population in which an all-dark coloration has now become common. Many people have probably seen all-dark gray squirrels in some areas, as an example. Down in the Caribbean, there are some islands where the normally bright yellow, white, and black bananaquits are completely jet black.

Although we don’t often think of it, the common city- and town-dwelling rock pigeon has an enormous variety of plumage variants quite unlike most birds. In fact, for many years, the Cornell Lab of Ornithology had a citizen science program that enlisted volunteers around the country to study the numbers and variants of rock pigeons. The British Trust for Ornithology has its own online survey of birds with aberrant plumage variations for bird enthusiasts in that country.

Such plumage variations always seem to capture the imaginations of those of who see them — perhaps because such aberrant colors can seem like they are a product of the imagination!