Effects of the adaptive background in the spectral sensitivity of budgerigars (Melopsittacus undulatus)

ABSTRACT The objective of this project was to study if the adaptive background affects the spectral sensitivity of budgerigars (Melopsittacus undulatus) tested in bright light. Two budgerigars were trained, by associative learning with food rewards, in dual choice tests to discriminate monochromatic light stimuli of nine different wavelengths from an adaptive background with ultraviolet light (UV) content. The tests were repeated as the UV-light was removed from the background. There were significant differences in the three wavelengths at the UV/blue-range of the spectrum between the two adaptive backgrounds (paired-sample t-test, n=8, p-value for 355 nm, 0.0025; 370 nm, 0.0017; 415 nm, 0.0058). The data are compared with predictions from the receptor noise-limited model proposed by Vorobyev and Osorio in 1998. Popular science summary: Colour vision of budgerigars (Melopsittacus undulatus) The beautiful colouration of bird plumage has been associated with colour vision, where birds have been found to be better than humans. The wavelengths of light that humans can see are between 380 and 740 nanometres (nm), this is what we call the visible part of the spectrum of light. Birds are able to see light of the same wavelengths as we do, but also light of wavelengths shorter than 380 nm (ultraviolet light, UV) that are invisible for us. Three types of pigments in the photoreceptors located in the retina mediate human colour vision while most birds possess a fourth type that gives them the capacity to see UV-light. Detection of UV-light helps birds to find food, to navigate and to find or attract a mate. An important part of vision is adaptation, which is a process that changes the visual sensitivity according to what we are exposed to. This process allows us to perceive for example yellow as yellow even when the light around us changes, such as at dusk and dawn. The objective of this project was to study how UV-vision of birds adapts to different backgrounds, using budgerigars as the model animal. Two budgerigars (called Bud and Hampus) were trained to associate light stimuli (narrow bands of wavelengths) with a food reward, and to differentiate the stimuli from two types of adaptive backgrounds: one containing white and ultraviolet light and a second only containing white light. Nine stimuli at specific points in the light spectrum were used for the tests with UV-light in the background and four stimuli at specific points at the light spectrum for the tests without UV-light. The objective was to find the intensity threshold when the light stimuli were just detectable by the birds in order to find their spectral sensitivity under the two types of background light. The obtained spectral sensitivities were compared to predicted differences based on visual modelling. The values obtained were very similar to the predictions when the background contained UV-light, but when the UV-light was removed, the budgerigars were not as sensitive as predicted. Despite this, budgerigars are still more sensitive to UV-light than other light. Studies have found that a great number of birds use UV vision to choose mates. This study gives indications of how well sexual UV-traits can be detected because the birds’ sensitivity was higher in the UV-range than in the visible region of the spectrum in both types of adaptive background. When the UV-light was removed from the background light, the spectral sensitivity of both birds increased significantly compared to the results with UV-light in the background. The spectral sensitivity is certainly affected by the composition of the adaptive background, as a big shift was evident in the spectral sensitivity for both birds. This means that the birds in their natural environment can see UV wavelengths better in front of a background of green vegetation or sand than in the skylight, since the photoreceptors receive less UV-light so they are more sensitive. Advisor: Almut Kelber and Olle Lind