The male in a pair of songbirds will usually be the brighter, more beautiful one. It’s a classic, textbook example of sexual selection. Simply put, males display and females choose because sperm is cheaper to produce than eggs. But the females of some species are just as colorful and ornamented as the males (see pictures below), and the evolutionary reasons underlying this diversity is still a mystery. 

According to a new Nature study, females tend to be more colorful when they’re in a monogamous relationship where the male helps take care of the chicks. And while sexual selection increases coloration in males, it actually has a greater effect in reducing coloration in females.

To explore why color differences between the sexes (called sexual dichromatism) vary among species, James Dale of Massey University in New Zealand and his colleagues developed a method to quantify plumage color in 5,983 species of songbirds. They digitally scanned images from the Handbook of the Birds of the World and then measured the RGB (red, green, blue) values for six patches on each sex of each species: nape, crown, forehead, throat, upper breast, and lower breast. Below are a pair of house finches (the male’s the colorful one).

Species with large bodies and long wings have increased coloration, the team found, and the difference between the sexes is less. Being larger likely reduces predation risk, so it’s less important to be camouflaged. Additionally, both males and females living in the tropics are more colorful than temperate species. Tropical birds tend to have smaller clutch sizes, and thanks to the low seasonality, there’s less need to migrate.

Next, to measure the strength of sexual selection, the team compiled an index that included factors like mating system and paternal care. A high scoring species, for example, would be one where males have the potential to mate with lots of females without having to take care of the offspring. As expected, in species where males are under stronger sexual selection, the males were more colorful. "But what we also found is that females tended to become much more drab when sexual selection on males was strong," Dale explains to IFLScience. "In fact the effect on females to get drabber was stronger than the effect on males to get more colorful."

Coloration in females is increased in species with high levels of female-female competition over reproductive opportunities. In other words, "females tended to be more colorful in species where males and females formed monogamous pair bonds with each other and where dad helped take care of the kids," Dale adds. 

^{Males are much more colorful than females in some species (e.g., top row, left to right: Baltimore Oriole, Red-legged Honeycreeper and Variable Seedeater). In many species however, females look the same as males and have just as dramatic plumage (e.g., bottom row, left to right: Blue-winged Mountain-Tanager, Crimson-collared Tanager and Chestnut-capped Brush-finch). Bill Holsten}

_{Image in the text courtesy of Bill Holsten.}