Almost all birds with bright red, orange, and yellow feathers or manes use a group of pigments called carotenoids to get their colors. However, these animals cannot produce carotenoids directly. They have to get them through the food from the plants they eat.
The exception to this rule is parrots, which have developed an entirely new way of producing color pigments called psittacofulvin.
Although scientists have known about these different pigments for some time, the understanding of the biochemical and genetic basis of how birds use them to distinguish themselves by color is less clear. But two recent separate studies of parrots and finches have yielded important insights into this mystery.
One study, published in Current Biology, was led by Daniel Hooper, and the other was led by Portuguese biologist Roberto Arbore and published in Science. Together they contribute to our understanding of how birds create their colours and exactly how these traits evolved.
A single enzyme
The two new studies involve large teams of international researchers. They have used the latest advances in genetic sequencing to investigate which regions of the genome (the complete set of an animal's DNA) determine the natural colour differences from yellow to red in parrot and finch families.
Remarkably, even though these two groups of birds create their colors using different types of pigments, the scientists found that they evolved in similar ways.
Arbore's research focused on the white-breasted lory (Pseudeos fuscata), a parrot native to New Guinea with streaks of feathers that can be colored yellow, orange or red.
The study found that the changes in feather colouration to yellow and red were linked to an enzyme called ALDH3A2. This enzyme turns the parrots' red pigments yellow.
When the developing feathers contain large amounts of the enzyme, they are yellow, and when they have less, they are red.
Scientists have found that the ALDH3A2 enzyme also explains color differences in many other parrot species, which have evolved from yellow to red regardless.
Two special genes
The white-winged tern (Poephila acuticauda) is a species of songbird native to northern Australia. There are two hybridising subspecies with different coloured beaks. One is yellow and the other red.
Most carotenoid pigments that birds can consume from their food are yellow or orange, so the birds' bodies must somehow change the chemical composition of the pigments after consumption to produce red colors.
Hooper's study looked at variation in this trait across the entire distribution of white-winged terns in the wild, as well as variation in the genomes of the birds measured. It turned out that beak color in these birds was mostly associated with two genes, CYP2J19 and TTC39B.
Together, these two genes stimulate the conversion of yellow dietary carotenoids to red.
In the Long-tailed Tern, the yellow coloration appears to be the result of mutations that turn off these genes specifically in the beak, retaining them in other parts of the body, such as the eyes.
As a regulator of light flow
Together, these studies show how colors can evolve in natural populations.
In both parrots and the finch family, the mutations responsible for the colour change from yellow to red have not altered the function of the relevant enzymes. Instead, they have affected where and when these enzymes are active.
Think of it as changing the lighting in a room by installing a light flux regulator on an existing light switch rather than removing the entire light fixture.
Scientists also show that in wild populations of parrots and finches, mutations of just a few genes can change the chemical structure of pigments significantly - enough to make the difference between red and yellow.
The key genes change the chemical structure of the pigment molecule through the action of an enzyme that adds just one atom of oxygen to the pigment. This changes its colour from bright red to bright yellow in parrots and vice versa in the finch family from bright yellow to bright red.
The miracle of nature
The evolution of colours has been in the crosshairs of science ever since Charles Darwin used them in setting out his theory of evolution by natural selection. The most obvious difference between the closely related bird species we see around us is their colour.
These two new studies show how a few genes and the addition of a single oxygen atom can change the course of evolution, creating a new form that looks so dramatically different.
If this improves the animal in an evolutionary sense - perhaps it looks more attractive to potential mates or stands out more - it could lead to the emergence of a new species. | BGNES
