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439Re: [wsbn] terminology question

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  • Bill Schmoker
    Dec 1, 2005
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      Hi Dona- those are good questions.  In the process of trying to understand a Steller's Jay that molted into pure white plumage (see upcoming article in the January issue of Colorado Birds), I delved into aberrant plumages.   As a disclaimer, I'm an earth scientist by formal training, and an amateur ornithologist mostly by diffusion.

      First, it is helpful to know that bird feather coloration can result from pigmentation or microscopic feather structures.  Birds can metabolize melanin pigments themselves, and acquire plant-produced carotenoid pigments from their diets (either directly from plants or through intermediate members of the food chain.)  Melanins produce earthy colors like black, gray, brown, dull yellow, etc.  Carotenoid pigments yield brighter colors like reds, yellows, pinks, and oranges.  A few bird families also can metabolize another family of pigments, the porphyrins, but here I'll ignore them.  Blue coloration in birds is not a result of pigmentation-- instead, microscopic structures in the feather barbs interact with light to produce coherent scattering in short (blue, violet, and ultraviolet) wavelengths.  Longer, backscattered wavelengths are absorbed by a basal layer of melanin granules in the center of each blue feather barb.  In short, blue feathers are good at reflecting blue light and also good at absorbing other colors.  Blue feathers only look blue in reflected light-- if you look through a blue feather, the transmitted light it will appear brown due to the melanin inside.  Green coloration is a combination of pigment and feather structure (with the notable exception of green porphyrins produced only by Turacos.)

      As I understand it, a bird with schizochroism (thus a schizochroic individual) lacks one pigment but other pigments appear to be normal.  As coincidence would have it, a friend emailed me a picture of a Yellow-rumped Warbler today that looked normal in the black, gray, and brown areas, but had no yellow where it should.  I would call this a schizochroic YRWA.  Why?  Apparently it could make melanin normally but wasn't metabolizing yellow carotenoid pigments in its diet properly.

      A similar aberration is when one color is missing but replaced by another color, usually yellow.  This is probably because yellow normally underlies the other color, thus, when there is a problem with the overlying pigment, the latent yellow shows through instead.  This condition is called Xanthochroism.  Examples I have seen include photos of a male Orchard oriole that looks normal except for a bright yellow face, a canary-yellow Evening Grosbeak with white wings and tail, and a Pileated Woodpecker with a yellow crest.  

      Leucism is a confusing term because general biology uses it differently than most birders.  In general biology, a leucistic individual lacks pigment (all or some.)  Thus, it is a broad term.  For example, you could say that a Polar Bear is leucistic (although completely normal), as well as a pure white Common Raven (completely aberrant) or a Common Grackle with a few white feathers in its tail (somewhat aberrant.)  Birders typically use the term more specifically to refer to a bird with less pigment (usually melanin) than normal, but not completely lacking pigment.  So a leucistic bird looks pale and washed out (often creamy or pale brown where darker colors should be), but not white.  This is also known as dilute plumage.

      The opposite of leucism is melanism, where a bird deposits more melanin than normal.  As you would expect, this results in a much darker bird, often dark brown or black where lighter colors or even white should be.  Our dark-morph buteos are great examples of this, although not aberrant.

      A partial albino bird completely lacks pigment in some or even most of its feathers.  The remaining feathers are colored, and usually the bare parts (eye, bill, feet) are normally colored, too.  

      Of course, a "true" or "complete" albino cannot make or deposit pigment, resulting in a pure white bird with pale (generally pink) bare parts and eyes. 

      Beware also that hybrids may exhibit unexpected colors in unexpected places, and that environmental elements like pollen, fruit juice, mud, pollution, or even iron-rich water can color birds in strange ways, too.  (A bright purple-faced Northern Mockingbird I once saw in a fruit-laden mulberry tree comes to mind...)

      Anyway, this is how I understand these terms- feel free to correct or add to them.

      Best- Bill

      On Dec 1, 2005, at 10:23 AM, Adams Lodge wrote:

      Wish I had a good bird to report but I don't. It's been snowing and blowing here and I haven't been out much. Only 4 juncos at my feeders, all slate-colored.
       
      What is the differences between schizochroism and leucism? How about partial albino and leucistic? What are the proper and accepted uses of these terms?
       
      Thanks,
      Dona Hilkey
       
       
       


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