What can cause birds to show weird color variations?
April 1, 2009
Some individual birds may look quite different than they appear in field guides. Often, there’s a very simple reason behind this: the bird is molting. For example, when American Goldfinch molt between their showy spring and summer plumage and their more drab winter plumage, they can look very unusual during the transition period. But birds, like other animals, can also exhibit naturally occurring color and pigment variations that can make ID difficult. For example:
- Albinism results from a genetic mutation that interferes with production of the pigment melanin. Unlike humans, animals without melanin may have other pigments, so they may still have some color. This means that a “true albino” bird may not necessarily be all white, and plumage patterns typical of the species, such as a mask or wingbars, may remain detectable. True albino birds are rare in nature because without protective pigments in the eyes, they may quickly become blind. Also, feathers wear out more quickly without pigments to provide structural support. Albinism may also be referred to as amelanism. Note, when you DO see a completely white animal with a lack of pigment everywhere—eyes, skin, hair, scales, and feathers—this may be due to a combination of albinism and other pigment defects (see below). These animals are generally referred to as “albino,” but that may not accurately describe the genetics behind their appearance.
- Partial albinos are much more common, and most birders eventually see at least a few of these individuals. The term partial albino describes a bird that is paler than normal, or a bird with irregular patches of pure white feathers. There are several different causes of partial albinism—it can be genetic, occur during development, happen as birds age, or after injury when new feathers lack pigments—and it is a useful general term as it can be impossible to determine why a bird is showing pale plumage without a genetic analysis. Birds with partial albinism may retain plumage patterns typical of the species, such as a mask or wingbars. Here are some reasons animals show partial albinism:
- Hypomelanism is a genetic mutation that causes a partial lack of melanin. A hypomelanistic bird will be paler than normal where melanin is expressed in the plumage.
- Leucism is not a genetic mutation, but rather describes defects in pigment cells that are caused during development. This may result in full leucism, where there is a reduction in all types of pigment. An animal with full leucism will appear paler than normal. Leucistic animals may also show irregular patches of white—this is referred to as partial leucism, and these animals are often referred to as “pied” or “piebald.” Because the development of the eyes occurs separately from other areas of the body, eye color in leucistic animals is not affected and will be normal in color.
- Sometimes with age or after an injury, white patches will appear on an animal. A feather may regrow after an injury and lack pigment. This will give the animal a partial leucistic appearance.
- Melanism causes birds to have an excess of dark pigmentation and is generally caused by a genetic mutation, but can also be a result of certain diets. Some species have a naturally occurring melanic form (or “morph”), such as the Red-tailed Hawk.
- Xanthochroism is a condition where individual birds of a given species may have yellowish or orange plumage instead of red. This may be caused by a genetic variation or by diet. House Finches are often reported as exhibiting xanthrochroism.
- Erythrism is condition where some individuals appear more reddish or rufous than others of their kind. For example, Eastern Screech-Owl and Ruffed Grouse often display erythrism and have commonly occurring rufous individuals. This condition is also caused by a genetic mutation or by diet.
If you think that you see an individual of a certain species, but the color isn’t quite “right,” keep these variations in mind and remember that size, shape, and behavior often help to identify a bird even when its plumage looks odd. Comparing the shape of a strange bird with other birds nearby can be very helpful as individuals often flock with others of their species.
Visit FeederWatch’s Unusual Birds page to learn more about color variations in birds.
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