Tetrachromacy, oil droplets and bird plumage colours

There is a growing body of data on avian eyes, including measurements of vi sual pigment and oil droplet spectral absorption, and of receptor densities and their distributions across the retina. These data are sufficient to pr edict psychophysical colour discrimination thresholds for light-adapted eye s, and hence provide a basis for relating eye design to visual needs. We ex amine the advantages of coloured oil droplets, UV vision and tetrachromacy for discriminating a diverse set of avian plumage spectra under natural ill umination. 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 ultravio let-sensitive peak is at 365 nm, generally shows a small advantage to the l atter, but this advantage depends critically on the noise level in the sens itivity mechanism and on the set of spectra being viewed.