What can cause birds to show weird color variations, such as being all or partly white, or unusually dark?
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 can cause both true albinos and partial albinos. Albinism usually results from a genetic mutation that interferes with production of the pigment melanin. True albinos are rare in nature because without protective pigments in the eyes, these birds quickly become blind. Also, feathers wear out more quickly without pigments to provide structural support. Partial albinos are much more common, and most birders eventually see at least a few of these individuals. Partial albinos have a pied appearance with usually irregular patches of pure white feathers. This form of albinism usually occurs during development, but may also happen after an injury when new feathers lack pigments. Some birds also develop stray white feathers as they age.
- Leucism is another plumage aberration caused by changes in pigments; leucistic birds are exceptionally pale all over. They produce smaller amounts of pigments in all their tissues, making the entire plumage look washed out while not being pure white. Plumage patterns typical of the species, such as a mask or wingbars, often remain detectable.
- 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.