On rare occasions you may observe a bird that has a familiar look to it--the right size and shape for a cardinal for example--but the color is not right. The absence or over-abundance of one or more pigments is the cause.
Melanistic color phase
Sometimes birds that are not normally black develop that way, and are called melanistic. This occurs when more than the normal amount of melanin is produced.
The Red-tailed Hawk on the left is in a typical plumage. The bird on the right is also a Red-tailed Hawk. In this bird more than the normal amount of melanin has been produced, resulting in a very dark Red-tailed Hawk (sometimes referred to as a Harlan's Hawk). Melanistic color phases of several hawk species are fairly commmon.
The melanistic Downy Woodpecker shown on the right was submitted by eBird participant Sam Galick. Melanistic woodpeckers are rare.
Albinistic color phases
In rare cases, a bird does not produce melanin at a normal level or in a normal pattern. The resulting color patterns are referred to as being albino (white), partially albino or leucistic. The color patterns can be the result of injury, poor nutrition or a genetic imbalance.
The bird on the left is an apparently leucistic Black Vulture. Remarkably different from a
typical Black Vulture (which is an all black bird), this bird appears to have very pale
grayish/brown feathers. The lack of a normal amount of melanin is probably the cause of this unusual color pattern, and is referred to as a partial albino or leucistic color phase.
Even more rare are birds that lack all three pigments, in which case the bird's skin and eyes look pinkish due to the hemoglobin in the bloodstream. This Turkey Vulture is an example of a complete albino. Usually all black, this Turkey Vulture is a pure, bright white, with pinkish face and legs. Photograph ©Heard Natural Science Museum and Wildlife Sanctuary
This piebald American Crow shows the isolated areas of white
associated with this color pattern. Such patterns are often
symmetrical. This pattern is also sometimes called partially albino.
Photo © Kevin J. McGowan
The absence of one or more pigment types can result in very strange looking birds. The white Pileated Woodpecker shown here was found during the search for the Ivory-billed Woodpecker. While melanin is mostly absent from the bird's feathers, carotenoid pigments are present and create the red in the bird's crest.
Other factors can also affect a bird's appearance. Feathers can become paler if bleached in the sun of the southwest, or become darker in areas with high air pollution. The berries they eat sometimes stain robins and waxwings.