Albinism and Leucism
Albinism is a genetic mutation that prevents the production of melanin (but not other pigments). Some colors come from pigments other than melanin, such as carotenoids. Albinism only applies to an absence of melanin; consequently, it is possible for a bird to be albinistic and still have color, although most consider true albinism to be an absence of all pigment.
Leucism refers to an abnormality in the deposition of pigment in feathers. There is some disagreement as to whether the condition is genetic or caused by pigment cells that were damaged during development. Whatever the cause, the condition can result in a reduction in all types of pigment, causing pale or muted colors on the entire bird. Or the condition can cause irregular patches of white, and birds with these white patches are sometimes described as “pied” or “piebald."
Albinistic birds have pink eyes because without melanin in the body, the only color in the eyes comes from the blood vessels behind the eyes. It is possible for a bird to be completely white and still have melanin in the body, as when a white bird has dark eyes. In this case the bird would be considered leucistic because the mutation only applies to depositing melanin in the feathers, not the absence of melanin in the body.
A third type of mutation that results in pied birds--birds that have white patches--is called partial albinism by some and leucism by others. The white patches are caused by an absence of pigment in some feathers.
To further confuse things, occasionally a bird will lose feathers in a close call with a predator. When this happens the new feathers sometimes grow in white and then change back to the normal color at the next regular molt. This kind of white coloring looks like leucism but is not and most frequently happens in the tail, causing a bird that lost its tail feathers to a predator to have an all white tail.
Melanism
Melanism is color variation referring to the excessive deposition of the pigment melanin. This results in feathers that are darker than normal. This color variation is far less common than leucism.
Xanthochroism
The color variation called xanthochroism refers to yellowish or orange pigments replacing normal coloration, usually red. FeederWatchers often report seeing strangely colored House Finches that they believe have xanthrochroism. However, House Finch coloration varies widely, and research has shown that most of the variation is diet-related rather than a pigment abnormality.
Yellow House Finches
FeederWatchers often report seeing strangely colored House Finches that they believe have xanthrochroism. However, House Finch coloration varies widely, and research has shown than most of the variation is diet-related rather than a pigment abnormality.
All male House Finches have the same potential for yellow, orange, or red coloration. Researchers who kept House Finches in captivity found that the red plumage was replaced by yellow plumage unless a carotenoid pigment was mixed in with their food during molt.
In the wild, three carotenoid pigments found in natural foods give House Finches their color. Beta-carotene produces yellow to orange colors, isocryptoxanthin produces orange colors, and echinenone produces red colors. Yellow House Finches are frequently seen in the southwest and Hawaii where natural foods are low in some of these carotenoids. In the east birds often feed on the high-carotenoid fruits of ornamental plants.
