Peculiar Bird Coloration
I decided to write a post about atypical bird coloration because of the strange-looking juvenile Cooper’s Hawk I saw at the south end of Tybee Island a couple of weeks ago. The bird likely demonstrated partial leucism because some of its feathers looked overly white. However, it was hard to tell if the bird’s feathers were actually white or simply appeared so because they were wet from the rain. Regardless, it gave me the idea for this post.
I am sure everyone has heard of and possibly seen albino animals. You also might have heard of a condition known as leucism. Both of these conditions are genetic mutations and both result in white or whitish feathers, but there is a subtle difference. Albinism is a condition whereby the bird cannot produce melanin, an important pigment that causes dark coloration. However, a bird’s colors can come from other pigments. Therefore, while most albino birds will appear all white, some albino birds can show other colors. Leucism, on the other hands, is the inability of the bird to deposit ANY pigment on its feathers. Leucism comes in two types – pied and paleness. Pied leucism is when some or all of the feathers are white due to complete lack of ability to deposit pigments. Paleness is when all feathers are paler than normal due to a reduction in ability to deposit pigments. If you see an all white bird in the wild, look at the color of the eye. Albino birds will have red or pink eyes because they cannot produce melanin in their eyes, causing them to appear the same color as the blood vessels behind their eyes. Because melanin blocks UV rays, albino birds have poor eyesight and therefore typically live short lives. Thus, you are far more likely to see leucistic birds in the wild, which have black eyes because this condition only affects the implementation of pigments on feathers. The most common type of leucism is partial pied leucism.
Another color variation is known as xanthochromism. This condition causes birds with red plumage to instead have yellow plumage in those areas. This can be a particularly tricky condition because it can cause birders to believe they are seeing a rare bird for an area when it might be a xanthochromatic individual of a regularly occuring species. This happened a couple of years ago in Georgia when a xanthochromatic Red-bellied Woodpecker was confused with its western counterpart – the Golden-fronted Woodpecker.
The next type of disorder is called melanism. Melanism is the over-production of melanin, causing birds to look far darker than normal.
Some birds are non-eumelanic, meaning they lack the pigment eumelanin. Eumelanin is responsible for blackish gray coloration in some birds.
Other birds are non-phaeomelanic, meaning they lack the pigment phaeomelanin. Phaeomelanin is responsible for chestnut brown or buff coloration in some birds. This condition typically causes birds to look dark gray because the only pigment left on the feathers which would normally contain phaeomelanin is eumelanin.
While there are many more genetic disorders that lead to plumage variations, this sampling should be more than enough to whet your appetite for knowledge. After all, the majority of these conditions are rare to come across (good thing too, for the birds sake).