The brilliant colors of birds have played a prominent role in the study of sexual and natural selection. In many species of birds males possess bright colors, while females are cryptic. This sexual dimorphism in plumage is traditionally explained as a consequence of variation in sexual selection (i.e., mating success). However, there are several reasons why this explanation may be inadequate. First, all comparative analyses to date have relied on subjective evaluations of plumage color and brightness by human observers, and it is now clear that birds see quite differently than humans. Not only do birds see in the ultraviolet (UV) spectrum to which humans are blind, they also see differently in the human visible spectrum. Unfortunately, there has not yet been a large-scale comparative analysis of UV reflectance in birds, so it is not known if UV signaling is widespread or if such signals commonly differ between the sexes. Second, recent studies of sexual differences in plumage color have found that evolutionary changes in plumage have been more common among females than males, suggesting that we have been focussing on the wrong sex. Indeed, there are many cases of bright colors in both sexes (e.g., parrots, tanagers, toucans) suggesting that selection has acted on females as well as males. Bright plumage in females may be favored by male mate choice or aggression between females over access to mates or food. In addition, natural selection may favor brightly colored females when both sexes defend a territory year-round or color patterns are involved in camouflage. Thus, other forms of selection besides sexual selection on males appear to be important in the evolution of avian plumage patterns.
The three year study proposed here will examine the evolution of avian plumage patterns and sexual differences in plumage color using objective measures of color (spectrometer measurements) taken from museum specimens. We will measure spectral reflectance of both sexes of the1031 species used in our previous study of avian dimorphism. By building on our previous work, this study will lead to significantly deeper understanding of signaling in birds and the evolution of plumage patterns, a subject that has enthralled and perplexed both biologists and lay persons for over 100 years. In addition, this study will provide opportunities for training students in comparative methods, enhance research facilities at UWM and produce a web site with spectral data of birds that can be used by other researchers and teachers.