Natural and sexual selection act on different axes of variation in avian plumage color.

The bright colors of birds are often attributed to sexual selection on males, but in many species both sexes are colorful and it has been long debated whether sexual selection can also explain this variation. We show that most evolutionary transitions in color have been toward similar plumage in both sexes, and the color of both sexes (for example, bright or dull) was associated with indices of natural selection (for example, habitat type), whereas sexual differences in color were primarily associated with indices of sexual selection on males (for example, polygyny and large testes size). Debate about the evolution of bird coloration can be resolved by recognizing that both natural and sexual selection have been influential, but they have generally acted on two different axes: sexual selection on an axis of sexual differences and natural selection on both sexes for the type of color (for example, bright or dull).

Multiple shifts between violet and ultraviolet vision in a family of passerine birds with associated changes in plumage coloration.

Colour vision in diurnal birds falls into two discrete classes, signified by the spectral sensitivity of the violet- (VS) or ultraviolet-sensitive (UVS) short wavelength-sensitive type 1 (SWS1) single cone. Shifts between sensitivity classes are rare; three or four are believed to have happened in the course of avian evolution, one forming UVS higher passerines. Such shifts probably affect the expression of shortwave-dominated plumage signals. We have used genomic DNA sequencing to determine VS or UVS affinity in fairy-wrens and allies, Maluridae, a large passerine family basal to the known UVS taxa. We have also spectrophotometrically analysed male plumage coloration as perceived by the VS and UVS vision systems. Contrary to any other investigated avian genus, Malurus (fairy-wrens) contains species with amino acid residues typical of either VS or UVS cone opsins. Three bowerbird species (Ptilonorhynchidae) sequenced for outgroup comparison carry VS opsin genes. Phylogenetic reconstructions render one UVS gain followed by one or more losses as the most plausible evolutionary scenario. The evolution of avian ultraviolet sensitivity is hence more complex, as a single shift no longer explains its distribution in Passeriformes. Character correlation analysis proposes that UVS vision is associated with shortwave-reflecting plumage, which is widespread in Maluridae.

Quantifying avian sexual dichromatism: a comparison of methods.

Recent advances in portable spectrophotometers have allowed researchers to collect quantitative, objective data on colour. There are few comparisons of the different methods used to summarize and analyse spectrophotometer data, however. Using colour data on over 900 species of birds, we compared three methods of calculating sexual dichromatism using spectrophotometer data. We also compared sexual dichromatism calculated from spectrophotometer data, in both the ultraviolet (UV) and bird-visible range, with human estimates of sexual dichromatism. We found that all three methods, principal component analysis, segment classification and colour discriminability, yielded essentially comparable estimates of dichromatism for our extensive sample of birds. Certain methods may be better suited to a particular study depending on the questions addressed and the specific colours examined. We found that human visual estimates of dichromatism were similar to spectrophotometer estimates of dichromatism in the bird-visible range; however, human visual estimates did not predict the extent of UV dichromatism. Therefore, the conclusions of previous studies that relied on human vision to assess sexual dichromatism should be reliable. It is not possible, however, to predict a priori whether a species exhibits UV dichromatism without spectrophotometer measurements.