Thursday, December 12, 2013
By HERB WILSON
In my last column, I discussed some instances of birds with aberrant plumages, including albinos. In today's column, I will continue the discussion of albinism in birds.
An albino bird is incapable of producing the pigment melanin. Melanin is responsible for most of the black and brown feathers in birds as well as the coloring of the iris of each eye and the color of naked parts of a bird, such as the legs. A true albino lacks melanin in the feathers, eyes and legs. It therefore has a pure white plumage, pink eyes (reflecting the blood vessels in the retina of each eye) and pink legs.
Albinism is relatively common in some groups of birds and rare or unknown in others. Of course, birds with red, yellow or orange feathers get their feather coloring from the pigments called carotenoids. The carotenoids are acquired from their diet rather than synthesized by the bird like melanin is. Albinos are only expected for birds whose primary pigment is melanin.
In Great Britain, a survey by B.L. Sage of over 3,000 records of albinism indicated that only 19 families of birds were represented. In decreasing order of incidence, albinism was most common in these families: thrushes, crows and relatives, swallows, weaver finches, starlings and finches.
Albert Gross conducted a similar survey of albinism for North American birds and found albinism to be more widespread. The 1,847 records were distributed among 20 orders and 54 families.
American robins were the most commonly reported (8.2 percent of all records) and house sparrows were next (5.5 percent of records). Waterfowl were also well represented. As Gross points out, these patterns may be misleading. Robins and house sparrows are common around our homes, so albinos of these species are more likely to be noticed than more secretive or human-averse birds. Hunters may preferentially shoot ducks with odd plumages.
Perhaps you have seen birds that show white feathering on only a portion of the body. Gross provided a classification to separate these variants from true albinos. He defined an incomplete albino as a bird that lacks melanin from the plumage, eyes or naked parts, but not all three regions as seen in a true albino. An incomplete albino might have pink legs and white feathers but dark eyes.
An imperfect albino has the melanin reduced or diluted but not absent in any or all of the three areas. My most memorable sighting of an imperfect albino occurred in March 2000 at Colby College. A flock of around 100 Bohemian waxwings descended to the ground beneath some ornamental fruit trees. Among them was one washed-out bird with tan body feathers rather than the rich gray-brown of a typical Bohemian. The black mask on the face was diluted as well to a dark brown. This imperfect albino was a striking bird with a beauty all its own.
The final category Gross described is partial albinism. A partial albino has the lack of melanin confined to a localized area. A black-capped chickadee is my most memorable partial albino.
In the winter of 1995-96, I was monitoring the use of sunflower seed feeders near Flagstaff Lake by a population of chickadees that I had color-banded so I could recognize individuals. I was not able to band all the chickadees that came to the feeders so I had no way to distinguish chickadee A from chickadee B if they were unbanded. However, an unbanded chickadee arrived one day missing all of its tail feathers. The bird had likely dropped its tail feathers (a phenomenon called fright molt) in a close encounter with a predator. The bird slowly regrew is tail feathers over the winter but they all came back pure white. The chickadee I called "White Tail" in my field notes was a partial albino.
Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at: