This time of year in the Eastern U.S., the birds are emerging in force, many flashing their brilliant reds—bright summer and scarlet tanagers, male cardinals in peak plumage or crimson male house finches.
It’s a pretty sight, but scientists have often wondered why some birds are red, since it seems the bright coloration would make them more conspicuous. Two independent studies recently published in the journal Current Biology have come up with some answers, including exactly what turns the birds red and what purpose it might serve.
“Red is usually an advertisement, but it's highly visible and likely costly, easily made out by predators,” Julia Schroeder, an ecology and evolution researcher at Imperial College, London, who was not involved in either study tells Jason Thomson at The Christian Science Monitor. “So we don't see it that often in animals.”
In one study, the researchers analyzed the genomes of three species, the red siskin of Venezuela, the yellow canary, and the “red-factor canary” a hybrid of the two bred in the early 20th century to produce a red-colored pet. According to Rachel Feltman at The Washington Post, the researchers isolated about 15,000 bits of the birds’ genome that could be involved in their red coloration. But one particular gene, CYP2J19, which is switched on in the skin and liver of the red birds jumped out.
Other duller bird species also have the gene, which usually produces some red in their eyes. But in red-feathered birds, it’s much more active and allows them to convert yellow carotenoids, colorful pigments found in plants, fruits, and vegetables, into ketocarotenoids which give their feathers a red glow.
A second, unrelated research team also zeroed in on CYP2J19 while studying zebra finches, a species whose males have a variety of beak colors ranging in shades of yellow to red. It turns out that the red-beaked finches carry CYP2J19, while the yellow-beaked zebra finches were a mutant strain lacking the gene, Thomson writes.
“The fact that we identified the same gene in two deeply divergent species suggests that this is likely to be very general in the bird world, and many species will use the same mechanism to produce red pigments,” Miguel Carneiro from the Universidade do Porto, Portugal, who co-authored the canary study tells Jonathan Webb at the BBC.
That explains how birds are red, but why are they red? Nick Mundy from Cambridge University who led the finch study tells Thomson they have an idea. The gene that helps create red ketocarotenoids is also associated with breaking down toxins. So it could be that the brighter the red, the better the bird is at detoxifying any nasty chemicals they consume, he says.
In other words, red birds are attractive to their mates not because they are pretty, but because they have strong livers.
Auburn University’s Geoffrey Hill, who led the canary study, says the discovery of the gene may have some commercial uses as well. “Red carotenoids are big business and they are getting bigger all the time,” he tells Ed Yong at The Atlantic. An artificial version of astaxanthin, a red pigment found in the eyes of some birds, is used as a food coloring that makes farmed salmon pink. Hill says learning more about the genes that make red birds red could lead to new classes of industrial colorants.