Among birds the world over, natural selection - during migration, breeding in subtropical locales and care of young - is as powerful as sexual selection, researchers have found.
Looking at nearly 1,000 species of birds, the team from University of Wisconsin-Milwaukee found that while males often have brighter feathers than females, the two sexes have come closer together in colour over time to blend into their surroundings and hide from predators.
"Our study shows that ecology and behaviour are driving the colour of both sexes, and it is not due to sexual selection," said Peter Dunn and Linda Whittingham, professors of biological sciences at UW-Milwaukee.
Although most studies of bird plumage focus on dichromatism, evolutionary change has most often led to similar, rather than different, plumage in males and females, the authors wrote in the journal Science Advances.
The team spent four years collecting data from 977 species of birds from six museums in the US and Australia.
They analysed the data, assigning each bird a colour score based on scales of brightness and hue.