Occasionally there are reports of abnormally colored bluebirds (or other species of birds) that are all or partially white. Scientific literature on the definitions of color abnormalities like leucism and various states of albinism is very confusing and inconsistent. Here's my stab at sorting it out (also see table of terms below.)
Albinism is a genetic (inherited) condition resulting in a complete lack of production of melanin pigmentation in the eyes, skin and feathers. Albinos are extremely uncommon.
Leucism is also a genetic mutation. Leucistic birds have dilute, paler/ whitish plumage overall. A faint pattern may be visible. Leucism is also uncommon, but is more common that albinism.
Some other color abnormalities may be due to disease, diet/malnutrition or injury, or other factors. for example:
Sometimes after an injury that damages feather follicles, the feathers that grow in may be white. When those feathers are replaced in the next regular molt, the new feathers may be a normal color.
Flamingos not provided with carotenoids in their diet turn white.
Feathers can become bleached in the sun.
Some birds develop stray white feathers as they age.
Other unusual pigmentation conditions that may appear in birds include abundism, erythrism, melanism, schizochroism, and xanthochroism. See glossary for details.
Some people use terms like partial/complete/true vs. incomplete or imperfect albinism and leucism. They also refer to pied or piebald and pattern mutation. As noted above, there is no clear agreement among authors on this subject. I'm using the three terms Albinism, Leucism and Pied/Piebald (see table below) for now, but I am NOT confident that my categorization is correct!
Genetics: Both parents must have the recessive albino gene for their young to be albinos. Inbreeding in small populations increases the odds that both parents will have the recessive gene and therefore produce albino young.
An abnormally colored Eastern Bluebird juvenile (notice color of eye, beak and legs) next to a normally colored sibling. Photo by Barbara Houston, taken in West Point area of Virginia. See more photos.
Photo by Amy Baldwin of North Carolina. Head and breast are buffy colored, probably because carotenoid pigments are present. All other feathers are white.
Photo of an abnormally colored bluebird by Karen Insull of Economy Boro, PA. It was hanging out with a half dozen other normally colored bluebirds. Note color of eyes, beak and legs. Note normally colored male and female adult Eastern Bluebirds below.
An abnormally colored Mountain Chickadee seen at a feeder in Big Bear, California (first two photos on left) compared to a normally pigmented Mountain Chickadee (on right.) Click on photo for larger images, taken Linda Violett.
Since each egg has its own set of genes, not all young in a nest will be albino or leucistic. Clutches with just one or two white bluebirds have been seen. One pair of bluebirds in Georgia had three nestings - in the first, one out of three fledglings were white; in the second three out of five were white, and in the third three out of five were white (Menaboni, 1947). (If a single recessive gene is responsible, the odds are 25% that the offspring of two parents with the recessive gene will have the mutation. Note that not all young in a nest always have the same parents, due to extra-pair mating and egg dumping.
Some researchers working with birds estimate that true albinism occurs in one out of 1,764 birds. (Source: MDC Online)
Survival: There are so few of these anomalies that it is hard to tell what the survival rate of albino/leucistic's birds is. Several factors could decrease their odds of survival.
They may be more vulnerable to predation because they lack protective coloration that helps them blend in.
Their feathers lack melanin, which provides strength and structural support, so they are more subject to wear and tear.
Melanin helps protect skin and eyes from overexposure to UV sunlight, so albinos are more likely to get melanomas and retinal damage, and may be blind. I would think it would be almost impossible for a blind bird to survive in the wild.
Steve Kroenke reported two leucistic Purple Martin fledglings being mobbed by normal martins.
Nicole Janke of GA reported a leucistic female that mated with a normal bird.
Terry Suchma of Fairview PA had a leucistic Purple Martin male in 2007 nest with an SY female. It returned the following year.
References and More Information:
Also see white and pink bluebird eggs (web page on Sialis.org) A female bluebird that lays white eggs should always lay white eggs. The color of the eggshell does not have anything to do with the coloring of the babies.
There are two forms of melanin: ?blackish eumelanin (lack of which can make a bird appear cinnamon-colored) and brown phaeomelanin (lack of which can make a bird appear silver-colored)
If carotenoid pigments are present, a bird could be almost all white with some red feathers, as in the case of a white Pileated Woodpecker with a red crest that was seen in Arkansas, and a breeding female Red-headed Woodpecker that had a red head and white body, with pink legs and feet and dark eyes (Rogers 1979)
The Audubon Society Encyclopedia of North American Birds by John K. Terres, Random House (1956-1980), has the following under Albinism: ...an albino ... bird has white feathers instead of the usual colors of its species (e.g., black or brown), and the white feathers may cover the bird wholly or in part, as there are various degrees of albinism....
The various degrees of albinism, classified by two geneticists, Mueller and Hutt (1941), and adopted by Nero (1954) and Pettingill (1956; 1970) are:
(1) Total Albinism: The rarest form, in which the bird has a complete absence of melanin (dark coloring pigment) from the eyes, skin, and feathers.
(2) Incomplete Albinism: Pigment is completely absent from either eyes, skin, or feathers but not all three.
(3) Imperfect Albinism: Pigment formation is partially inhibited (reduced) in eyes, skin, or feathers but pigment is not totally inhibited in any.
(4) Partial Albinism: The commonest form; complete or partial albinism within local parts of the body which may involve certain feathers only; it is often symmetrical and each side of the bird may show white feathers in the same pattern
.... Under the subheading Abnormal Colors under Colors of Feather: Another abnormal color phenomenon of birds is called schizochroism, a term applied to an abnormally pale, washed-out bird whose paleness results from an absence of one of the pigments normally present in its plumage....
Like American Robins, bluebirds are in the Thrush family. "According to Gross (1965a), the American robin, with 152 records of albinism (8.22% of all records), and the house sparrow, with 104 records (5.53% if all records), led all other birds in highest incidence of albinism. Among the 1,847 cases of albinism reported by Gross, only 7% were total, or complete albinos.” (from the Terres encyclopedia) Note from Bet: This could be because there are higher numbers of robins and House Sparrows around places where people might see them.
See also leucism in Harrison, J. M. (1964b), a condition closely allied to albinism.
Sibley uses the following categories:
The Sibley's guide discusses the subject briefly under the category of"Aberrant Plumages"
"In albinism and partial albinism all or some feathers are pure white. Partial albinism often follows feather groups, so that white spectacles or an entirely white head might appear. Certain species, including Red-tailed Hawks, Willet, gulls, American Robin, blackbirds and grackles, seem more prone to albinism than others."
In leucism (also referred to as dilute plumage) normal patterns are visible, but all plumage is paler than normal, usually pale, creamy brown."
Dr. Walden, herpetologist, indicates that in albinism, chromatophore cells are present but cannot produce pigments? In leucism, he says chromatophore cells are not present. Other sources say pigments are dilute, or deposition of pigments is reduced or abnormal.
Albinism is believed to be due to a single (non-sex-linked) gene (Bruckner 1941; Sage 1962).
Another definition of leucism is "complete loss of a particular pigment, or all pigments, in feathers but not in soft-parts. It may be as slight as a single white feather or as pervasive as an all-white bird with normal eyes, bills and legs (Buckley 1982, also Joseph R. Jehl, Jr., Leucism in Eared Grebes in Western North America, The Condor, 87:438-441, 1985)
Other references I want to check:
Deane, R. 1876, 1878, 1879. Albinism and melanism among North American birds. Bull. Nuttall Ornithol. Club, Vols. 2, 3, and 5.
Gross, A.O. 1965. The incidence of albinism in North American birds. Bird-Banding 36:67-71.
Nero, R.W. 1954. Plumage aberrations of the Red-wing (Agelaius phoeniceus). Auk 71:137-55.
Sage, B.L. 1962. Albinism and melanism in birds. Br. Birds 55:201-22.
TABLE OF TERMS
(sometimes referred to as "true" or "complete" or hypomelanism)
Leucistic or Leucism (Leukism) (sometimes referred to as "partial or incomplete albino")
Pied or Piebald
(sometimes referred to as partial leucism)
No melanin (completely absent)
Some (reduced/ diluted/ abnormal deposition?)
Localized or incomplete
Absent from iris. Eyes appear pink. May be blind
Normal. Dark (not red/pink) - iris may be pale gray/blue or normal color
White or much lighter than normal (look pale or "bleached," "diluted" or whitewashed). Symmetrical (same on both sides of body). Plumage patterns typical of the species, such as a mask or wing bars, often remains faintly detectable.
Irregular or isolated/blotchy white patches (symmetrical or asymmetrical. The entire head on a Bald Eagle is piebald, thus the term "Bald")
Skin, feet, beak
Pink or reddish (because blood shows through)
Normal color? See photo by Houston - beak is yellow, feet are lighter colored.
Confusion is a word we have invented for an order which is not yet understood
- Henry Miller
Unusual individuals in a population always seem to arouse human curiosity....
- Rogers et al, 1979
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