In macro-fauna, eye-catching colors typically serve two purposes: to woo potential mates, or to signal that the creature is not a snack and will in fact poison you. (In microbes, coloration serves different purposes, as we have previously covered on the blog. See here and here for examples.) This is where parrots diverge – and get embroiled in connections to microbiology.
Parrot feathers contain special pigment molecules called psittacofulvins, which are linear polyene chains that come in red and yellow varieties (psittakós is Greek for "parrot" and fulvus is Latin for "reddish yellow"). Of the approximately 10,000 extant species of birds on the planet, parrots are the only ones to produce psittacofulvins within their rainbow of hues. In addition to red and yellow feathers, which contain their respective colored psittacofulvins, green parrot feathers also contain yellow psittacofulvins (plus some melanin). Meanwhile, blue feathers appear blue entirely due to structural coloration – the interplay of light with microscopic structures to "produce" color – rather than pigmentation (as we covered previously here, almost all blues in nature are a "trick of the light"). Other colors, such as black and brown, owe their hue simply to melanin, while white is the absence of melanin.
It's the red psittacofulvins that are special. Like melanin, these confer resistance to bacterial degradation: red parrot feathers, when exposed to the common feather-degrading bacterium Bacillus licheniformis, degrade at a slower rate. The long chain-like structure of these pigment molecules confers an additional safeguard against bacterial degradation, bolstering the protection already provided by the keratin in all feathers. It's interesting that this protective property does not apply to yellow psittacofulvins, and yellow parrot feathers degrade at the same rate as pigment-less white feathers. Red and yellow versions of the pigment differ by their terminal aldehyde and terminal carboxyl, respectively (Figure 2).
The parrot pigments are unusual in another way. In other birds, red, yellow, or orange feathers owe their coloration to carotenoids, which are obtained through their diets. The color of flamingos is thanks to carotenoids gleaned from the pink shrimp they eat, which themselves acquire the carotenoids from the algae that they eat. Similarly, songbirds like cardinals and orioles eat bright red berries and colored seeds, and so the intensity of their colors can vary depending on what food they find. How psittacofulvins end up in a parrot's feathers is a different story, however. Rather, these parrot pigments are made in-house, produced by a polyketide synthase from the MuPKS gene cluster (a PKS not in a bacterium, a fungus, or a plant, but in a vertebrate!). Blue coloration in wild-type green feathers can be attributed to a single T-to-C point mutation in MuPKS. Presumably the point mutation hamstrings the MuPKS enzyme's ability to produce the pigment.
Other birds, including chickens and crows, do in fact possess homologs of MuPKS. But these are expressed at levels hundreds to thousands of times lower than in parrots and do not contribute to a yellow or red appearance. Despite this, the chicken MuPKS, when expressed in yeast, can produce yellow pigment. So perhaps an ancestral bird possessed the enzyme, but it was only in the parrot lineage that it became employed in feather coloration. (Note: parrots are distinguished from other bird families by their hook-shaped beaks and their zygodactyl feet, with two toes pointing forward and two toes pointing backward.)
Why is it that this pigment is only highly expressed and produced in parrots? That is an open question and anyone is welcome to speculate, but maybe there is a connection to parrot species' tendency to inhabit very humid climates in the tropics, since feather-degrading bacteria tend to be more active under humid conditions.