Atd Bennett

Tetrachromacy, oil droplets and bird plumage colours

Abstract There is a growing body of data on avian eyes, including measurements of visual pigment and oil droplet spectral absorption, and of receptor densities and their distributions across the retina. These data are sufficient to predict psychophysical colour discrimination thresholds for light-adapted eyes, and hence provide a basis for relating eye design to visual needs. We examine the advantages of coloured oil droplets, UV vision and tetrachromacy for discriminating a diverse set of avian plumage spectra under natural illumination. Discriminability is enhanced both by tetrachromacy and coloured oil droplets. Oil droplets may also improve colour constancy. Comparison of the performance of a pigeons eye, where the shortest wavelength receptor peak is at 410 nm, with that of the passerine Leiothrix, where the ultraviolet-sensitive peak is at 365 nm, generally shows a small advantage to the latter, but this advantage depends critically on the noise level in the sensitivity mechanism and on the set of spectra being viewed.

Plumage Reflectance and the Objective Assessment of Avian Sexual Dichromatism

Assessment of color using human vision (or standards based thereon) is central to tests of many evolutionary hypotheses. Yet fundamental differences in color vision between humans and other animals call this approach into question. Here we use techniques for objectively assessing color patterns that avoid reliance on species‐specific (e.g., human) perception. Reflectance spectra are the invariant features that we expect the animals color cognition to have evolved to extract. We performed multivariate analyses on principal components derived from >2,600 reflectance spectra (300–720 nm) sampled in a stratified random design from different body regions of male and female starlings in breeding plumage. Starlings possess spatially complex plumage patterns and extensive areas of iridescence. Our study revealed previously unnoticed sex differences in plumage coloration and the nature of iridescent and noniridescent sex differences. Sex differences occurred in some body regions but not others, were more pronounced at some wavelengths (both ultraviolet and human visible), and involved differences in mean reflectance and spectral shape. Discriminant analysis based on principal components were sufficient to sex correctly 100% of our sample. If hidden sexual dichromatism is widespread, then it has important implications for classifications of animals as mono‐ or dimorphic and for taxonomic and conservation purposes.