When bluebirds fight, bet on the bluest.

Among male eastern bluebirds, their blues, along with bird-visible ultraviolet colors in their plumage, give a pretty good indication of which males make the toughest competitors, according to a study of nesting birds.

Bluebirds may be symbols of happiness to us, but males compete fiercely for nesting holes. Lynn Siefferman and Geoffrey Hill of Auburn University in Alabama took advantage of these struggles to test ideas about how plumage color serves as a means of communication.

Researchers had previously observed in other bird species that feather pigments can make males more appealing to females. In some cases, the pigments signal the health of a mate and other measures of fitness.

That gorgeous bluebird blue, however, poses new questions because it doesn't come from a pigment. Instead, the color is a trick of the light bouncing off intricate structures on feathers. Researchers have wondered whether such structure-based color could also signal the fitness of a male.

The researchers set out birdhouses and watched to see which males triumphed in competitions to take up residence there. Feathers plucked from the winners turned out to have more-intense structural coloring than the losers' did. The more colorful males also successfully raised more offspring. Thus, the color could be an indicator of which bird to wager on in male-male competitions, the researchers report in an upcoming Animal Behaviour.--S.M.