Intraspecific variation in avian colour vision, and the effects of diet on avian visual physiology

• Main Author: Knott, Benjamin
• Published: University of Bristol 2009
• Subjects: 598.14
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.685420

Avian vision is highly developed and complex. Avian retinas contain rod and double cone photoreceptors, and four classes of single cones. Single cones are sensitive to red, green, blue, and violet/ultraviolet wavelengths, producing, it seems, tetrachromatic colour vision. All cones contain an oil droplet, rich in carotenoid pigments. These droplets act as selective filters, substantially modifying light detected by the photoreceptor. Using dietary carotenoid manipulations, the effects of carotenoid availability on oil droplet absorbance properties were tested in two species: the zebra finch and the crimson rosella parrot. Microspectrophotometry was used to determine whether these manipulations affected oil droplet carotenoid concentration, a change that could alter the colour discrimination ability of birds, with consequences for individual behaviour and ecology. Oil droplet concentration was determined by measuring the cut-off wavelength at which no more light is transmitted. Significant increases concentration were found in carotenoid supplemented birds, but only in one droplet type not affecting colour vision. Analysis also revealed the effect sizes resulting from the manipulation were dependent on retinal location. These results reveal a response to dietary manipulation that potentially preserves spectral tuning of colour vision, and could also provide insights into the development of these droplets in the retina. In the zebra finch, additional measures of plumage spectra and immunocompetence were taken to assess the trade-off of carotenoid resources used in vision with those needed for other carotenoid dependent systems, but these studies were inconclusive. Intraspecific variation in visual physiology is a phenomenon known in fish and mammals, but currently unreported in birds. The oil droplets of green and blue budgerigars were measured to test for a relationship between plumage pigmentation and droplet absorbance. No relationship was found, but the spatial variation in oil droplet absorbance within the retina was notably similar to the spatial variation in crimson rosella. The visual pigments and opsins of the crimson rosella were characterised using micro spectrophotometry and molecular techniques, which revealed unusual features in RH 1 and RH2 opsins that were previously unknown in any vertebrate opsin.