The roles of receptor noise and cone oil droplets in the photopic spectral sensitivity of the budgerigar, Melopsittacus undulatus

Individual budgerigars (Melopsittacus undulatus) were taught to detect narrow bands of wavelengths under ambient illumination of known spectral composition. Because the cone pigments of this species of bird have been identified and data on carotenoid absorbance present in the cone oil droplets are available, predictions of the Vorobyev-Osorio equations can be calculated with reasonable confidence. Based on more than 27,600 individual choices made by several birds at 10 wavelengths, the photopic sensitivity (i.e., color thresholds) of these birds is found to be consistent with the hypothesis that threshold discrimination of colored targets is limited by receptor noise And that high sensitivity to near-ultraviolet wavelengths is in harmony with the relatively small number of ultraviolet cones present in the retina. The pronounced fine structure of the sensitivity spectrum is caused by the absorption of cone oil droplets. Under natural sunlight, containing more energy in the near-ultraviolet than is present in artificial indoor lighting, the birds' peak of sensitivity in the ultraviolet should be much less prominent than it is in laboratory experiments.