Male and female birds often have strikingly different coloration, likely a result of sexual selection for choosing mates. In some canaries, the difference is driven by a surprisingly simple molecular mechanism, according to a new study by Malgorzata Gazda and colleagues. Their findings add to what is known about the genetics of sex-specific coloration in birds, and could aid in the study of how a sexually selected trait like dichromatism has evolved over time. The researchers looked at the phenomenon in mosaic canaries, which are a cross between the dichromatic red siskin and the common canary, where males and females have similar coloration. Through careful genetic crossing, genomic mapping and other techniques, Gazda et al. found that the dichromatism of mosaic canaries is linked to the gene BCO2, which catalyzes an important step in the degradation of carotenoid pigment in feathers. (Carotenoids produce red, orange and yellow colorations in many birds.) In the less colorful female birds, they found, upregulated BCO2 degrades carotenoid pigmentation. In a related Perspective, Nancy Chen discusses the researchers' findings regarding the varying role of BCO2 in dichromatism among other birds such as the European serin and the house finch.