Leucism -A Genetic Mutation

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default Leucism -A Genetic Mutation

Post  FinchG on Wed Oct 19, 2011 4:12 pm

Albinism is a genetic mutation that prevents the production of melanin in the body. Leucism is a genetic mutation that prevents melanin from being deposited normally on feathers. Since the white patches on birds have no melanin, birds with these patches have been said to have partial or imperfect albinism.

Leucistic pale Northern Mockingbird by Clayton Gascoigne, Hendersonville, Tennessee

More recently, however, scientists are clarifying the definitions and pointing out that since birds with white patches do have melanin in the body, they cannot be albinistic.

Therefore the white patches are caused by a defect preventing normal deposition of the melanin. And since leucism is a deposition problem, it makes sense that birds with white patches would be leucistic.

Consequently leucism comes in two main varieties — paleness, an equal reduction of melanin in all feathers; and pied, an absence of melanin in some feathers creating white patches.

Interestingly, albinism only applies to an absence of melanin. Since some colors come from other pigments, such as carotenoids, it is possible for a bird to be albinistic and still have color. Leucism, on the other hand, applies to all pigments. It is also possible for a bird to be completely white and still have melanin in the body. In this case the bird would be considered leucistic and would have dark eyes because the mutation only applies to depositing melanin in the feathers.

Albinistic birds have pink eyes because without melanin in the body, the only color in the eyes comes from the blood vessels behind the eyes.

Leucistic pied Black-capped Chickadee by John Matthews, Salem, Oregon
Leucism is relatively unusual in birds, and albinism is rare. From 2000-2006, Project FeederWatch participants reported fewer than 1000 leucistic birds. Given that participants report about 5.5 million birds each season, the percentage of leucistic birds being reported is very small.
Leucistic: Pied House Finch by Jeffery Anderson, Espaniola, New Mexico
Typically birds with abnormally white feathers do not survive long because they are so much more visible to predators. Those that do survive may have trouble attracting a mate. Consequently, the mutated genes that cause albinism and leucism are less likely to be passed on to a new generation. If you are ever fortunate enough to see one of these oddly plumaged birds, consider yourself lucky! And if your sighting occurs during the FeederWatch season, report the bird using an Unusual Bird Form online, or send us a note with your paper data forms at the end of the season.
Leucistic pied Fox Sparrow by James Shea, Castro Valley


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