One enzyme has a key role in determining whether parrots’ rainbow plumage emerges in vibrant reds, yellows or greens, a new study shows.

Parrot feathers flaunt an array of colors, with yellow, red and orange hues made possible by pigments called psittacofulvins that are found only in the vibrant birds (SN: 3/15/05). Blue tones come from nanostructures on feathers — which can appear green when combined with yellow psittacofulvins — and melanin controls black, grey and brown colors (SN: 2/7/24).

But how psittacofulvin pigments shift shades was unclear.

An enzyme called ALDH3A2 alters the chemical composition of the pigments, which are made primarily of carbon molecules, to control color in developing feathers, researchers report in the Nov. 1 Science. Through a chemical process called oxidation, ALDH3A2 turns red aldehyde molecules into yellow carboxylic acids.

Previous research showed that a different enzyme called PKS makes red aldehyde-based psittacofulvins, says Joseph Corbo, a biologist and neuropathologist at Washington University School of Medicine in St. Louis. The new study reveals that ALDH3A2 comes in as a second step, switching reds to yellows.

Corbo and colleagues turned to the dusky lory (Pseudeos fuscata), a parrot species that comes in red and yellow varieties, and rosy-faced lovebirds, (Agapornis roseicollis) which have both red and green feathers, to crack the color code.

A combination of chemical and genetic analyses of the parrots revealed that ALDH3A2 is more active in yellow and green feathers than in red ones, suggesting that the enzyme helps feathers color swap. What’s more, the ratio of aldehyde- to carboxylic acid-containing molecules can create a sliding scale of colors, the team found. For instance, equal ratios of red and yellow molecules should produce orange feathers, Corbo says.

Yellow feathers topped with blue nanostructures makes rosy-faced lovebirds’ feathers green, while blue with red psittacofulvins could create purple, Corbo says. When factoring in melanin, parrot feathers have “many different ways of mixing and matching these different types of pigments to achieve sometimes unusual colors.”

Parrot psittacofulvins are chemically very similar to carotenoids, pigments that other birds such as cardinals get from their diet to give them a bright red color. It raises the question of why parrots use psittacofulvins, Corbo says. “Are these molecules better than carotenoids in some way?” One hypothesis is that psittacofulvins create feather colors more durable than pigments that need to be replenished with birds’ diets do, meaning brilliant parrot plumage never fades.