"In many bird species, the redder the male, the more successful it is at finding mates."
Scientists say they have solved a long-standing mystery about what makes some birds red -- a color that, when found in beaks and feathers, shows strong sex appeal.
The genes affecting coloration belong to a wider family of genes involved in detoxification, said the study in the journal Current Biology.
That means redness may be a sign of a robust, quality mate who can easily cleanse harmful substances from his body.
"In many bird species, the redder the male, the more successful it is at finding mates," said co-senior author Joseph Corbo, an associate professor of pathology and immunology at Washington University in St. Louis.
Birds like canaries and zebra finches eat seeds, fruit and insects that provide yellow pigments, known as carotenoids.
Some birds are able to convert the yellow molecules to red ones -- known as ketocarotenoids -- using enzymes that are active in the eyes of red and yellow birds, as well as in their feathers and skin.
"It was quite a surprise that the same genes are involved both in seeing red colors and making red coloration," said co-author Nick Mundy from Cambridge's Department of Zoology.
"Our findings fill this gap and open up many future avenues for research on the evolution and ecology of red coloration in birds."