Color Abnormalities in Birds
Birds' plumage can sometimes vary due to an excess, or a deficiency, of color pigment in some or all of the feathers. Pigments are compounds that determine the color in birds' feathers.
Albinism is a genetic abnormality that results in a complete lack of pigmentation in the feathers, eyes, skin, and bill. The result is a completely white bird with a pinkish tint on unfeathered areas of the body caused by blood vessels showing through colorless skin.
The outward appearance of leucistic individuals can vary considerably. "Leucinos" may appear virtually pure white but still possess "normal-colored" eyes and skin; they may possess only a single white feather but otherwise appear normal; or they may exhibit partially white plumage anywhere in between these two extremes. Many older publications refer to this condition as "partial albinism" but, the correct term is leucism.
A bird with a decreased amount of pigmentation resulting in a pale or washed-out appearance has a condition, referred to by Dr. P.A. Buckley, an avian geneticist, as dilution.
The presence of excessive amounts of melanins (dark pigments) cause light-colored feathers to vary from brown to black. Melanism is less common than albinism and leucism.
Occasionally some birds (e.g. black-capped chickadees) are seen showing an entirely white tail. While leucism could be responsible for this, it can also be caused by the loss of the tail to a predator. The new "replacement" tail occasionally grows lacking pigment.