You have to be careful about many of the birding books and sites. Most PhD's in ornithology know very little about chromatophore biology and they throw words around carelessly. To be fair, ornithologists are not physiologists, cell biologists or pathologists and have most of their education in behavior, ecology and taxonomy. So they can perhaps be forgiven for not really understanding the finer points of pathological conditions and the need for consistency within the scientific community that deals with these issues. Hobbyists are really bad about using terms incorrectly and that comes from much misinformation on the net and from reading books by those who are not really qualified to make a diagnosis of albinism or leucism (leukism). There are several sites with really weird definitions out there that have no pathological basis. One site that has been brought to my attention:
This one does a fairly good job at pointing out the inconsistencies in the birding literature regarding leukism.
Another problem with the bird literature is the superficial examination of the animals themselves with no real scientific scrutiny. The tendency to throw the word leucism (leukism) around is rather frightening. Any animal with a messed up patch of feathers that have gone white is called leucism by these people. Just look at the Cornell folks:
SCARY for anyone concerned with accuracy and precision, the way scientists are supposed to be.
The problem with this "bird brained" approach to the question is they are examining feathers. These are structures composed of keratin and they have no idea whether or not there is a lack of pigment, reduction of pigment or if there is an impairment of pigment transfer to the keratin. Transfer impairment would constitute a whole new pigment disorder not leukism, which has already been established as a defect involving the survival or migration from the neural crest of chromatophores. To date this has not been examined. What is more, is that many birds called leukistic are not really leukistic. They have pigment on most of their feathers, they just have patches of white. That is not really classical leukism. Pattern mutations, piebaldism, or some other disorder should be considered before the diagnosis of leukism is applied. To make a diagnosis of a disorder without understanding the basic pathogenesis is at the least VERY BAD FORM, and MALPRACTICE at worst!
Furthermore all you have to do is look at a calico cat as a good example. Due to the X chromosomes having different color genes you get two colors, right? No, you get three. One color on calicos is white. Tuxedo cats also have black and white. This is not leucism. It is a pattern mutation. The abnormal patchy distribution of white in the feathers of birds may be something similar, and this must be ruled out before they can be called leukisitc.
Regarding pigments, other sites which are not so good, I will not mention. But several of you have asked about another birding term, that really has no pathological authority. Xanthochroism is an odious term to be frank about it. It really has no real biological basis. It often is seen in psittacines and in aquarium fish. The cause is generally a lack of the melanins and other pigments that cause the yellow pigments to be all that is left, so yellow predominates. There is a problem with this term. First the term means yellow skin. Not a good term for birds, since we are talking about the feathers not the skin itself. Also it is a lack of pigments that causes the problem, not excess of yellow. To understand the pathogenesis, it is necessary to understand the proper name to give the condition. By the way, xanthism is also used for this and that is a really poor choice.
Here is a list of color abnormality definitions so you can see why this is a problematic condition to name.
- Erythrism (Erythristic) - excessive production and deposition, or distribution of red pigments (orange possibly).
- Anerythrism/Anerythristic - lack of production of pigments in the darker orange to red range.
- Hypoerythrism/Hypoerythristic - reduction in the amount of darker orange to red pigments so that the appearance of this color is largely absent except for traces or appears "washed out."
- Xanthism (Xanthic) - excessive production and deposition, or distribution of yellow pigments (orange possibly).
- Axanthism/Axanthic - lack of yellow and lighter orange pigments, depending on the point in the pigment cascade, this mutation can also cause corresponding anerythrism since erythric pigments (drosopterins) appear to come from the more yellow pteridines biochemically.
- Hypoxanthism/Hypoxanthic - reduction int he amount of yellow or lighter orange pigments so that the appearance of this color is only found in trace amounts or appears "washed out." This may also result in hypoerythrism since the red pigments appear to be made from the yellow pteridines.
- Melanism (Melanistic) - excessive producution and deposition, or distribution of melanin pigments (may be orange if pheomelanin to black if eumelanin).
- Amelanism/Amelanistic - lack of melanin production. At least three basic forms are possible, though whether all forms have been observed is questionable. 1) amelanism where the chemical cascade is defected before eumelanin and pheomelanins take separate biochemical routes, resulting in a complete lack of melanin production. 2) aeumelanism - where only eumelanin production is blocked. 3) apheomelanism where only production of pheomelanins is blocked.
- Hypomelanism/Hypomelanistic - condition resulting in the reduced production of melanins. At least three types are possible by restriction of production at the initial stages of melanin production, at the eumalnin production cascade or at the pheomelanin cascade.
- Iridism (Iridistic) - excessive production and deposition, or distribution of iridophore platelets (this is, as yet, only a theoretical condition).
- Aniridism/Aniridistic - (again theoretical - I have not heard this reported) lack of the formation of refractile platelets in iridophores.
- Hypoiridism/Hypoiridistic - (theoretical) reduction in the number of refractile platelets formed in iridophores.
So if the condition in psittacines is caused by reduced melanin for example it is really a hypomelanism or amelanism not xanthism or xanthocroism. Furthermore, many birds have blue in the feathers as a structural color, that means it is not caused by pigments but by structural design of the feather causing the reflection of blue light back toward your eye. If yellow pigment and blue structures are combined then you will perceive green. In this case if the feathers turn yellow it is a structural change in the feather and that is another defect entirely. That may be a good condition to use xanthocroism for, or perhaps another name (I am open to suggestions here).
A Word on Blue
Blue is generally a structural color and is the result of the interaction of iridophores and other chromatophores. There should be a note here that some species possess other forms of chromatophores. Leukophores (leucophores) are described in fish, but they are really iridophores that have platelets reflecting white back at the observer's eye. Being a bit of a lumper, I really do not consider leukophores a separate cell line, but some people do. I must also admit to and point out a bit of hypocrisy here, since I tend to talk about xanthophores and erythrophores, but lump iridophores and leukophores.
Among the other cells out there are some that have been called cyanophores. No they do not produce lethal cyanide. Cyan = blue. "Bluephores" then are cells that would contain blue pigments. This has thus far been unusual. Most animals do not have blue pigments, but use iridophores and other chromatophores to produce blue. However, that does not mean that cyanophores do not exist. They have been demonstrated in fish, and I strongly suspect they are present in cephalopods and would be surprised if they are not. Are they present in amphibians and reptiles? No, not so far as I can find, but that does not mean they are not present in some species and just have not been described. I do not look to find them in any known reptile species (though I would be pleased if I was surprised), but hope to find a paper one day where they have found them is some rain forest amphibian.