Cynic said:
Also, I would say that colorization may be an indicator of health. If perhaps the woman on the right was malnurished, she might look pale, as so.
Similarily an inadequate amount of nutrients might take it's toll on a bird's plumage.
Certain illnesses can cause such discoloration.
Color also stands out, so if a bird evolved colorful plumage, it would have more sucess in passing on it's genes. Think of it as advertisement.
Ah, but there's more to it than that! As an extract from :-
http://www.biochemist.org/news/page.htm?item=15425 will tell you;
.........................."Animals, like birds and fishes, commonly use biochromes like carotenoids to acquire red, orange or yellow coloration, but McGraw and Nogare found that these compounds are not responsible for the red colours found in the parrot species they sampled.
The researchers used high-performance liquid chromatography (HPLC) to survey the pigments present in red parrot feathers. McGraw and Nogare collected and analyzed samples from 44 parrot species that have red feathers. Overall, there are some 350 species of parrots, 80 percent of which have red in their plumage.
They found a suite of five molecules, called polyenal lipochromes (or psittacofulvins), which colour parrot plumage red in all of the species studied.
"We've uncovered a system where all red parrots use the same set of molecules to colour themselves," McGraw said. "It is a unique pigment found nowhere else in the world. We are fascinated at how parrots are able to do this.
"The fact that there is a single set of molecules unique to and widespread among parrots, suggests that it is a pretty important evolutionary novelty, and one we should carefully consider when we think about why parrots are so strikingly colourful," McGraw said.
He added that an interesting aspect of the five polyenal lipochromes that provide the red in parrots, is that the pigment is found only in the bird's feathers and nowhere else in the body of the bird, indicating that parrots manufacture these molecules internally and directly at the maturing follicles of the growing, colourful plumage.
In addition, these pigments may play a valuable role in maintaining the health of parrots. McGraw cites an independent study on the parrot pigments that suggests that they can act as anti-oxidants to quench free radicals and potentially protect cells and tissues in the body from oxidative damage.
Now, McGraw says, he's interested in learning more about the connection between the red colours and anti-oxidants within and among parrot species, as well as "to specifically explore the balance of naturally and sexually selected costs and benefits to becoming colourful. "...........................
