It’s best to see red if you’re a bird

When it comes to sexual selection in birds, red means go – and a pair of papers has figured out why.

Two genetic studies have linked red colouration in birds to a specific gene that encodes a red-making enzyme, and may also be linked to detoxification, explaining why red birds are more attractive to their potential mates.

The studies, published this week in Current Biology, also identify this particular gene as the force behind some birds species’ ability to produce red colouration from yellow pigments in food.

The use of red in the bird world among feathers, skin and beaks, is common to different species to attract potential mates or threaten rival birds. Interestingly, the colour red was created within the bird’s metabolic system.

One of the studies, led by Miguel Carneiro of Universidade do Porto in Portugal, analysed the canary’s genome to discover the mechanisms behind this process.

Red canaries first appeared a century ago, when twitchers bred a yellow canary with a red siskin, creating a hybrid species.

In this study, researchers compared the genomes of yellow canaries, red siskins and the hybrid species to identify genetic differences that may be responsible for the red colouration.

“To produce red feathers, birds convert yellow dietary, pigments known as carotenoids, into red pigments and then deposit them in the feathers,” explains Carneiro.

“Birds also accumulate these same red pigments in one of the cone photoreceptor types in their retina to enhance colour vision.

“We discovered a gene that codes for an enzyme that enables this yellow-to-red conversion in birds.”

An enzyme created by a specific gene cluster known as P450 was highly concentrated in the skin and livers of red canaries, suggesting it may be responsible for the transformation of yellow pigment.

In a separate study, this gene cluster was also identified by researchers in the UK and Sweden as a red-maker, when they compared the genomes of zebra finches with a red beak with those carrying a yellow beak mutation.

Yellow-beaked zebra finches showed multiple mutations in the P450 gene cluster and almost no trace of the red-making enzyme in their beak.

The gene set responsible for the creation of the enzyme is also linked to detoxification, explains Staffan Andersson, a biologist at the University of Gothenburg in Sweden and co-author of the zebra finch paper.

“In sexual selection, red colour is thought to signal individual quality and one way it can do this is if the type or amount of pigmentation is related to other physiological processes, like detoxification,” Andersson says.

“Our results, which link a detoxification gene to carotenoid metabolism, may shed new light on the debated honesty of carotenoid-based signals.”

Armed with this new knowledge, researchers are now keen to investigate the instance of red colouration in other species, to compare the genetic processes behind them, with the aim of understanding a wider variety of genetic traits in birds.