http://en.wikipedia.org/wiki/Xanthochromism explains it but it seems to be from a north american perspective and Greenfinch is not directly mentioned. But it does say that it could also be due to diet.
PS, I never knew that engineers knew such long words, let alone spell them8-P B
The photographs can also be seen here, where the difference between the presumed xanthochroistic bird is easier to appreciate. But is it really xanthochoism? To me it just looks as though there's a lack of green pigment.
The ignorance of some people is beyond belief.
The green color is "build" by yellow (lipochrome) x black/brown (melanin). If the bird is missing the black part in the melanin the leftover result is "brown".
See:
http://www.ariafromabirdcage.com/Color.htm
or
http://www.colourcanaries.co.uk/melanin_canaries.html
At the latter you can ask the experts, if you need another explanation.
The ignorance of some people is beyond belief.
The green color is "build" by yellow (lipochrome) x black/brown (melanin). If the bird is missing the black part in the melanin the leftover result is "brown".
See:
http://www.ariafromabirdcage.com/Color.htm
or
http://www.colourcanaries.co.uk/melanin_canaries.html
At the latter you can ask the experts, if you need another explanation.
Source: http://www.farbenkanarien.homepage.t-online.de/fk/fk_home.htmAus Untersuchungen in anderen Tierklassen wissen wir, dass durch Mutation des für die Synthese des Melanins verantwortliche Gens (hier s+ genannt) aus dem ursprünglichen Schwarz nur noch Braun entsteht.
Das bedeutet, dass die Melaninvorstufen sich nicht vollständig bis zum Schwarz entwickeln, sondern diese Polymerisation bereits bei einer braunen Vorstufe beendet ist. Die so entstandenen braunen bis maximal dunkelbraunen Melanine werden in der Feder eingelagert. Schwarze Anteile im Gefieder werden nie gebildet.
We know from studies in other animal classes that by mutation in the gene responsible for the synthesis of melanin (here called s +) from the original black only brown develops.This means, that the melanin precursors do not fully develop until the black, but this polymerization is already finished with a brown preliminary stage. The brown to maximally dark-brown melanin, developed in such a way, are stored in the feather. Black portions in the plumage are never formed.
Jan-Paul, sorry for my poor English in thise case. I wish I would be fluent.
A rather amateurish translation. Hope you can unterstand it: