In a worldwide study of the evolution of bird plumage coloration, a team of scientists led by Prof Peter Dunn of the University of Wisconsin-Milwaukee found that while male birds often have brighter feathers than females, the two sexes have come closer together in color over time to blend into their surroundings and hide from predators.
The Lesser bird-of-paradise (Paradisaea minor) was one of 977 species examined by Prof Peter Dunn and his colleagues at the University of Wisconsin–Milwaukee in a worldwide study of the evolution of bird colors. Image credit: Reza Pratama / CC BY-SA 2.0.
“Although most studies of bird plumage focus on dichromatism (a form of sexual dimorphism in which males and females differ in color), evolutionary change has most often led to similar, rather than different, plumage in males and females. Our study shows that ecology and behavior are driving the color of both sexes, and it’s not due to sexual selection,” the researchers said.
Prof Dunn and his colleagues collected data from 977 species of birds (around 10 percent of all bird species) from six museums in the United States and Australia. They looked at six specimens – three males and three females – of each species.
The scientists analyzed the data, assigning each bird a color score based on scales of brightness and hue. They examined plumage color in relation to ten measures of natural and sexual selection.
“When the sexes became more similar in color, they did so for reasons of natural selection. When the color gap increased, it had more to do with sexual selection,” the researchers said.
“A lot of research has focused on how plumage color is related to mating success, especially in males, so this should hopefully get researchers to think more about how color affects survival, especially predation and foraging success, in both sexes,” Prof Dunn said.
The team also found that male birds with multiple mates actually tend to be duller in color than their female counterparts. Male red-winged blackbirds, for example, can have up to a dozen mates but are less colorful than their consorts.
“The reason for this is that males in these species often have a lot of black plumage,” Prof Dunn explained.
The findings appear in a paper published online March 27 in the journal Science Advances.
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Peter O. Dunn et al. 2015. Natural and sexual selection act on different axes of variation in avian plumage color. Science Advances, vol. 1, no. 2, e1400155; doi: 10.1126/sciadv.1400155
