Color variations defined
Since the article was so short, decided to just type it up! I don't know whether the LA Audubon newsletters are online, but I'm sure they wouldn't mind my sharing this with the Forum. Obviously, any typos found are mine. Enjoy!
Katy
Bolsa Chica's White Willet
By Charles T. Collins
"Western Tanager"
(Los Angeles Audubon Society newsletter)
November/December 2002, Vol. 69, No. 1
One of the most attractive qualities of birds is their bright and varied coloration. I doubt that there would be as many birdwatchers if birds were all drab "little brown jobs." The colorful appearance of birds is achieved by a variety of pigments being deposited in the skin and growing feathers.
The two commonest types of pigments are the melanins and carotenoids. The melanins include eumelanin which is responsible for the darker black and gray colors, while phaeomelanin is responsible for some of the lighter browns and tans and erythromelanin the richer chestnut-red colors. Melanins are synthesized by the birds and deposited as granules in the skin and feathers.
The carotenoid pigments include carotenes (red-orange), xanthophylls (yellow and orange), and carotenoids acids (reds). They are softer, non-granular, compounds which give birds their many shades of pink, red, orange and yellow. They produce some of the most intense colors such as those found in Scarlet Ibises and tropical tanagers and also the faint rosy blushes of the breeding season Elegant Terns and Ross's Gulls. These pigments for the most part cannot be synthesized by birds and must be obtained from plant or animal material in their diet and then deposited, often chemically unaltered, in feathers and soft parts.
In addition to pigment colors the physical phenomenon of light scattering is responsible for blue in feathers and skin as well as white or albinistic feathers. The highly metallic iridescent colors of many hummingbirds are caused by interference phenomena, as is also true in an oil film on water, but they also depend, in part, on underlaying melanin pigment layers.
Many of us have at one time or another seen a bird with abnormal coloration, particularly albino or partially albino birds. However, it is often more complex than a simple presence or absence of pigments. In June and July 2002 what appeared to be a white or largely white Willet was seen and photographed at the Bolsa Chica Ecological Reserve. Analysis of this bird is a good starting point for categorizing the several types of plumage abnormalities which occur in birds. This bird, as shown in the accompanying photos, was not a true albino, which requires a complete loss of pigment in both feathers and soft parts. Where there is a loss of pigmentation in the feathers but not the soft parts it is considered leucism. The spectacular photo of a white hummingbird with dark eyes and bill featured in the July-August 1994 "Western Tanager" would thus be a prime example of complete leucism. The term partial leucism would be used when there is only a partial to nearly complete loss of pigments from the plumage with white feathers occurring symmetrically or non-symmetrically over various parts of the body. Most people would simply call these partial albinos.
Less commonly encountered is melanic leucism where only the melanic pigments are lost and carotenoid pigments remain. An example of this is a specimen of a nearly all white Yellow-headed Blackbird in the Cal State University, Long Beach collection which lacks all of the black coloration but retains some of the yellow in the head region. It would certainly have been a spectacular bird to see in the field! Carotenoid leucism would be the opposite, where the carotenoid pigments are lacking but the melanins remain. A Yellow-rumped Warbler lacking the yellow throat, pectoral spots and rump but otherwise normally colored would be an example of this. Both of these cases refer to situations where these two types of pigments occur in different parts of the plumage.
Yet another category of abnormalities is called schizochroism. It involves situations where one pigment overlays another in the same feather. Melanic schizochroism occurs when there is a loss of the phaeomelanins (the browns) resulting in a more uniformly gray bird, or the loss of the eumelanins (the dark browns or blacks) resulting in an all tan or "fawn" colored bird. Melano-carotenoid schizochroism would describe the loss of one or the other of these two types of pigments where they normally are present in the same area. I have seen a specimen of a Red-winged Blackbird where the absence of some of the black pigment showed the red (which should be confined to the epaulets) to be much more widespread but normally masked in other areas by the denser melanins.
Carotenism describes several abnormalities involving the carotenoid pigments. These include changes in the distribution or amount of these pigments present as well as the shift from red to yellow often seen in our local House Finches. Extreme cases of all yellow birds, probably due to melano-carotenoid schizochroism, is found in some cage birds, particularly parrots, which are referred to as 'leutinos' by aviculturists.
Melanism is due to an excess of the eumelanins resulting in abnormally dark plumage as is occasionally seen in 'dark morph' Red-tailed Hawks and jaegers. An extreme case of this would be an all black bird which normally would have shown other colors as well.
Now, back to the white Willet of Bolsa Chica. The presence of some melanin pigments on the tip of the bill and in the wing feathers (partially delimiting the characteristic wing stripe of Willets) rules out this being a true albino. Partial leucism would probably adequately describe the condition in this bird. However, examination of the color photographs of this bird show some yellowish color to the base of the bill and upper legs, which would normally be obscured by overlaying melanin pigments. Thus this bird also exhibits a degree of melano-carotenoid schizochroism as well.
All of these different categories of plumage abnormalities may seem like splitting hairs (splitting feathers?); we could just continue to call them partial albinos or dark phase birds and be done with it. However, being aware of the actual basis for the diverse colors and color patterns, as well as the several kinds of abnormalities we may encounter, should serve to sharpen our observational skills and our overall enjoyment of birds and birding.
In writing this I have relied heavily on the chapter on 'Genetics' by P.A. Buckley in the book 'Diseases of Cage and Aviary Birds' by M.L. Petrak. I am indebted to Mike Bowles for the use of his superb photos of Bolsa Chica's white Willet which prompted this whole article.
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Dr. Collins is an emeritus professor in the Department of Biological Sciences at California State University, Long Beach, where he continues to teach classes in Ornithology and Behavioral Ecology. His primary research interests are the biology of swifts, Island Scrub Jays and coastal breeding terns and skimmers.
