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 pigeon's 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.

Tetrachromacy, oil droplets and bird plumage colours

Publisher Summary Birds can see ultraviolet (UV) light because, unlike humans, their lenses and other ocular media transmit UV, and they possess a class of photoreceptor, which is maximally sensitive to violet or UV light, depending on the species Birds have a tetrachromatic color space, as compared to the trichromacy of humans Birds, along with some reptiles and fish, also possess double cones in large numbers and a cone class This chapter discusses a range of behavioral experiments, from several species, which show that UV information is utilized in behavioral decisions, notably in foraging and signaling Removal of UV wavelengths affects mate choice even in species that are colorful to humans These studies emphasize that avian and human color perceptions are different and that the use of human color standards, and even artificial lighting, may produce misleading results However, genuinely objective measures of color are available, as are, importantly, models for mapping the measured spectra into an avian color space

Ultraviolet Vision in Birds

SEXUAL selection is one of the most actively studied areas of evolutionary biology1–3, and ever since Darwin1 birds have been probably the most popular taxon for testing the predictions about colour variation. Humans have been used to assess 'colour', an approach which may be flawed4,5 as many birds see in the ultraviolet (to which humans are blind), and have at least four spectral classes of retinal cone cells (humans have only three). Here we report experiments on zebra finches which test the hypothesis that the ultraviolet waveband (300–400 nm) is used in avian mate-choice decisions. We found that the ultraviolet is used, and that it probably contributes to hue perception. This finding may have wide implications for future studies of avian sexual selection and colour, and supports one hypothesized function of avian ultraviolet vision, the role of which is largely unknown.4,6,7

Ultraviolet vision and mate choice in zebra finches

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 animal's 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 mor...

https://www.jstor.org/stable/10.1086/303160

Plumage Reflectance and the Objective Assessment of Avian Sexual Dichromatism.

We investigated the signaling function of blue plumage in male blue grosbeaks (Guiraca caerulea) to determine if structurally based coloration may act as a reliable signal of quality to conspecifics. Blue plumage results from the microstructure of feather barbules rather than from pigment granules, and thus it is possible that structurally based plumage ornaments may function differently from sexually selected ornamental coloration that is pigment based. The plumage of male blue grosbeaks reflects maximally in the blue-ultraviolet range, so most variation in plumage coloration among males is invisible to human observers. In previous research, we showed that increased area of blue plumage on the body is associated with a shift in the wavelength of maximum feather reflectance toward the ultraviolet and with high intensity of light reflected at that maximum, and that extreme expression of the male ornament is condition dependent. These observations suggest that blue plumage may be an honest advertisement of male quality. We tested this hypothesis in a wild population of blue grosbeaks. We quantified male quality in three broad categories. (1) Physical condition was assessed from subcutaneous fat deposits, ectoparasite load, and body size. (2) Territory quality was assessed from territory area, prey abundance, and predation risk. (3) Paternal investment was assessed from male feeding rate. We found that the bluest males have the largest body size, maintain the largest territories with the greatest prey abundance, and feed nestlings in the first nest of the season at the highest rates. We conclude that structurally based plumage coloration functions as an honest, intraspecific signal of quality.

/pdf/structurally-based-plumage-coloration-is-an-honest-signal-of-1xws7zr9p8.pdf

Structurally based plumage coloration is an honest signal of quality in male blue grosbeaks

Many diurnal birds possess a set of four spectrally different types of cone photoreceptor. The spectral range of vision is extended into the UV by a UV receptor. We have investigated the spectral properties of plumage colours including the UV by reflection spectroscopy. Some basic general principles of colour vision indicate that plumage colours are tuned to avian colour vision.

Biological aspects of bird colouration and avian colour vision including ultraviolet range

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Biological aspects of bird colouration and avian colour vision including ultraviolet range

Biological aspects of bird colouration and avian colour vision including ultraviolet range