|4/4/2018 1:55:00 PM|
By Libby HillA robin is one of the most welcome harbingers of spring, promising flowers and other beautiful birds just around the weather corner. Suppose, however, a bird poses like a robin, sounds like a robin, is the typical size of a robin, runs on the ground and cocks its head listening for a worm like a robin, but instead of looking like a robin with a typical rich rust-colored belly and dark back, it has white splotches all over its body, or even a white head. What bird is this?
Recently, Info@ensbc.org (information at Evanston North Shore Bird Club organization) received a number of questions about whether the back yard robin-like bird with the mottled white feathers is actually a robin. The easy answer is “Yes.” These birds are known as leucistic robins. The explanation is more complex, and scientists still do not fully understand how bird colors and patterns are produced.
Feather color on birds, as well as hair and fur color on mammals, is basically genetically controlled by melanin. Genes carry the code or blueprint for the production of melanin. The main gene involved is MC1R (short for melanocortin-1 receptor) gene. To remind readers of Biology 101: Genes have two “alleles.” According to 222.yourdictionary.com, “alleles are pairs or series of genes on a chromosome that determine the hereditary characteristics.”
Individual birds inherit sets of genes that control the color of their plumage. If one or both of the alleles in the melanin-producing gene undergoes a mutation, melanin will not be deposited as expected in a feather.
The result is leucism.
Melanin is manufactured in a bird’s body from amino acids and is deposited in feathers while they are growing. Feather color is controlled by the percentages of two types of melanin. Eumelanin is responsible for black, brown and greyish colors and for dark caps, bibs, masks, bars, spots and stripes. Phaeomelanin is responsible for the earthy tones in some of the most beautifully marked birds with rufous, chestnut, golden and yellowish hues.
In leucistic birds, intense colors, especially red, orange or yellow, may show up brightly, but normally brown or black feathers instead show up pale or white.
The creation of melanin is a complex process and can break down in several ways: an inability to produce or deposit melanin in feathers, a lack of one type of melanin, or a partial loss of one or both types of melanin. One study found that “although in most cases the origin of the mutation causing leucism is unknown, Ellegren et al. (1997) found that a population of Barn Swallows, Hirundo rustica, exposed to high levels of radioactivity at their breeding site around Chernobyl, Ukraine, in the years following the nuclear catastrophe there in 1987 had an unusually high frequency of leucism.”
Some leucistic birds can lose all the pigment in their feathers and may appear pure white. Leucistic birds differ from albino birds. In albinism, the genetic mutation prevents melanin from getting into a bird’s entire body, not just its feathers. Albinos are all white and have pink eyes, whereas leucistic birds, although their feathers may be all white, have normal-colored eyes.
Mutation is for life and will be passed on to succeeding generations. Breeders of birds from parakeets to pigeons have long taken advantage of melanin mutations. By breeding preferred mutations in color, they have created unusual birds of a feather.
Other factors influence the color of feathers, among them the bird’s diet, feather structure, and hormones. Carotenoids – the pigments in plants that are responsible for the red, yellow and orange color in many fruits and vegetables – play a part in feather color. Carotenoids come from a bird’s food and are deposited in feathers during their growth. After the bird eats a carotenoid-containing berry, the carotenoids may move through the circulatory system directly to feathers, bill and legs. In other cases, they are chemically changed by the amino acid and fatty acid compounds in the bird’s body.
Yellow pigments are chemically transformed to red in Northern Cardinals and Scarlet Tanagers. Observers found that Cedar Waxwings that ate the fruits of the non-native species of honeysuckle, Lonicera morrowii, while the tail feathers were growing, ended up with orange tail bands instead of the usual yellow.
Creating and transporting coloration requires energy and resources on the part of the bird. If the transport of carotenoids is disrupted, the result is a more drab color of feathers, bills or skin, but not leucism.
Structural coloration, not pigments, explains the brilliant glittering hues of hummingbirds and the blue of blue jays. Structural coloration is the result of the interaction of light with the microstructures of a feather and can involve melanin or carotenoids.
The same hormones that signal to a bird that it is time to migrate and to breed also create seasonal and age feather colors. Feather color and pattern change from nestlings to juvenals to adults. The color of winter male goldfinches, for example, resembles that of the female and the young bird.
Don and Lillian Stokes, who have written many popular books about birds, speculate that “Birds with leucisim or albinism may have a harder time in the wild, as they may be more visible to predators and not as attractive to a potential mate.” The actual effects of leucisim need much more research.
A red-green color-blind person can identify a bird by its pattern, behavior, size, shape, location and sound, regardless of color. It is important to engage all the senses when identifying a bird, especially one as curious-looking as a leucistic robin.
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