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.