Seeing a bird with abnormal white feathers or white plumage prompts questions about how rare or common such sighting may be. We will consider two survey results and review leucism and albinism to determine how rare these two abnormal plumages occur in wild birds.
In a nutshell:
Birds with abnormal plumages are very rare, and both leucism and albinism are easily recognized and more commonly reported than other abnormal plumages. According to survey data, leucism and albinism occur at a rough estimate of 1 in 30,000 birds. Leucism is more common than albinism. Survey data suggest that of 100 birds that show abnormal plumages, 82 are leucistic, and only three are albinistic. The rarity of albino birds in nature is attributed to detrimental physiological factors.
Without a reference to the ratio of abnormal versus normal plumaged birds, it is difficult to determine the rarity of leucistic and albino birds.
Fortunately, two separate surveys help answer the questions:
- How rare are leucistic and albinistic birds (abnormal plumages)? and
- Is leucism more common than albinism?
How rare are leucistic and albinistic birds?
Cornell Lab of Ornithology’s Feederwatch Program receives approximately 5.5 million reports of birds visiting the bird feeders of program participants each year.
Program participants reported 1,650 birds with unusual plumages visiting their feeders between 2000 and 2007, resulting in about 236 unusual-plumaged birds per year. Birds with unusual plumage generally refer to individuals with leucism or albinism. Other abnormalities are not excluded but are expected to be even rarer.
Only 236 of the 5.5 million birds reported each year had leucism or albinism, making up a tiny proportion of birds with abnormal plumages. In other words, only about 1 bird in 30,000 has leucistic or albinistic plumage.
Based on these results, leucism and albinism are very rare occurrences among birds.
How common is leucism in birds compared to albinism?
The British Trust for Ornithology (BTO) surveyed the frequency of abnormal bird plumages. A simple questionnaire posted online allowed the public to report birds with unusual plumages.
“If you have seen a bird with unusual plumage in your garden, please complete our simple online questionnaire.”
Results of the BTO survey
In total, 3000 records of abnormal bird plumages were submitted, covering 58 species. Birds as small as wrens (0.3 to 0.4 ounces) and as large as common buzzards (1 to 3 pounds) were reported.
|Total Records Submitted||Reported as leucistic||Reported as albinistic||Reported as melanistic||Reported as other abnormal plumage|
This survey indicates that leucistic birds are much more common than albino birds, at a ratio of 82% to 3% of the 3,000 records submitted.
Thus, of 100 birds with abnormal plumages, 82 are expected to be leucistic, and only three are expected to be albinistic.
Across species, leucism and albinism occur at varying rates
In the survey, Blackbirds (Turdus merula) exhibited abnormal plumages more frequently than all the 58 species covered in the survey. The following species with a high frequency of abnormal plumages include the house sparrows (Passer domesticus), jackdaws (Corvus monedula), and carrion crows (Corvus corone).
These results provide evidence that abnormal plumages are more prevalent in some species than others.
The fact that blackbirds, house sparrows, jackdaws, and carrion crows are the most common species in the regions where the survey took place may suggest skewed numbers towards those species. However, survey managers normalized the data by dividing the frequency of each species reported weekly in the area.
This data normalization makes the frequency of abnormal plumages across species comparable.
There is also some confusion because species that exhibit a high frequency of albinism or leucism have dark plumages compared to species with naturally pale or white feathers. Although this is a valid concern, the ratio of leucistic to albinistic plumages still holds, at least, among the birds with dark plumages.
Birds with natural gray, pale, or white feathers might have been undercounted as leucistic birds with only a few white feathers are more difficult to separate as abnormal in the overall pale plumage of those species.
Why is leucism more common than albinism?
Leucism, the absence of feather color, is caused by multiple mechanisms, including nutritional deficiencies, hereditary mutations, and progressive graying of the feathers.
- Nutritional deficiencies: Poor nutrition can affect cells that produce carotenoid and melanin pigments. When regular production of color is disrupted, colorless, white, or gray feathers are produced.
- Genetic mutations: A genetic mutation can also cause leucism, which can be passed onto offspring. “Leucistic genes” are always recessive and normal-plumaged adults can carry leucistic genes without showing signs of leucism. When adult birds with recessive leucistic genes mate, the resulting offspring are likely to exhibit leucistic plumages. Leucistic genes can skip generations without being expressed in offspring.
- Environmental Factors: Genetic mutations that result in leucism can also be caused by extrinsic environmental factors. Studies have shown that there are more leucistic birds in areas with high levels of mutagens (contaminants).
Bird leucism and habitat use, age, and sex
A study on leucism in birds found that habitat type used by birds, age, and sex are associated with the frequency of leucistic plumages in birds.
As mentioned above, researchers found that there are more leucistic birds in cities than in non-urban areas suggesting that mutagens and contaminants in the city may contribute to a greater frequency of leucistic birds.
When it comes to the age of birds, there was a positive association between the presence of white feathers and the age of the birds. Older birds were more likely to have white, grayed, or diluted feathers than younger birds.
The study also found that male birds exhibited leucism more often than females. However, this association was relatively weak.
Why albino birds are very rare in nature
Genetic mutations cause albinism, which is caused by the lack of tyrosinase in pigment cells. Albino birds do not produce carotenoid or melanin pigments, so “all” feathers are pure white.
The bare parts of an albinistic bird, such as its beak and legs, are colorless. The beak and legs may show shades of yellowish, pink, or flesh color depending on these parts’ natural color.
The color of the eye (iris) may be the most distinctive and diagnostic characteristic that distinguishes an albino from a leucistic bird. Albino birds have colorless eyes that look pink or reddish due to the blood vessels inside the eye.
True albino birds have short lives.
Most albino birds do not live long enough to reach sexual maturity and reproduce. The majority of albino birds are thought to die soon after they fledge.
Those that reach adult age live under such stress that they are energetically unable to find a mate and initiate a breeding attempt.
Albino birds live a short life due to physiological reasons
Generally, albino animals can have poor vision since their eyes or iris (the colored section of the eye) lack pigment. The albinos’ retina (the thin layer of light-sensitive nerves at the back of the eye) also has an unusual development called foveal hypoplasia, in which the optic nerve is ‘misrouted’ between the eye and the brain.
Animals with albinism can also suffer from more common vision problems, such as strabismus (misaligned eyes), nystagmus (uncontrolled eye movements), and refractive errors (farsightedness, nearsightedness, and astigmatism).
These conditions are common to most albino animals and are expected to affect albino birds similarly.
Albinism can affect birds ability to fly.
In albino birds, the reduction of pigment causes their feathers to weaken and wear more readily. Flight feathers that are worn down or broken can affect a bird’s flight.
Birds need to fly to find food and escape predators.
Birds of unusual colors tend to be viewed as easy prey by predators, such as hawks, that target them more often than birds of normal colors.
Albino birds are rare partly because of physiological and behavioral barriers associated with albinism, meaning that they die young. Therefore the rate at which they occur and are observed in nature is very low.
Based on the information gathered and survey data, abnormal plumages in birds are very rare. Both leucism and albinism are characterized by the presence of white and discolored feathers. While leucistic birds can function nearly normally, albinistic birds face several physiologic barriers leading to very short lives or lives under strenuous conditions. Physiological differences between leucistic and albinistic birds help explain why leucistic birds are more common than albinistic ones.
- Factors associated with leucism in the common blackbird Turdus merula. Lucía Izquierdo,Robert L. Thomson,José I. Aguirre,Alazne Díez-Fernández,Bruno Faivre,Jordi Figuerola,Juan Diego Ibáñez-Álamo. Journal of Avian Biology – 02 July 2018.
- British Trust for Ornithology – Garden Birdwatch.
- The Cornell Lab Ornithology – Project Feederwatch.
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