Bird Vision: Spatial acuity and colour discrimination in bright and dim light

Abstract: Birds use vision to guide navigation, foraging and mate choice, and studies of the limitations in bird visual perception are most helpful for the understanding of bird ecology. Here, I present four studies of bird vision in bright and dim light. Paper I is a quantification of how sensitive colour vision modelling is to variation in data, such as the spectral sensitivity of photoreceptors and receptor noise, which are used as model parameters. In Papers II and III, I have studied budgerigars (Melopsittacu undulatus), which are strictly diurnal, and Bourke’s parrots (Neopsephotus bourkii) that are diurnal but also active during twilight. Paper II shows that the intensity limit of colour vision and the optical sensitivity of single photoreceptors are similar in both species. Both parrots need more light to see colours (between 0.1-0.4 cd/m2) than other vertebrates (e.g. 0.01 cd/m2 for humans). In Paper II, it was also found that Bourke’s parrots have larger eyes and retinas with a higher density of rods compared to budgerigars. In Paper III, I investigated whether this gives Bourke’s parrots enhanced spatial vision in dim light. This does not seems to be the case, the spatial acuity is similar in both species at light intensities between 0.08 cd/m2 and 73 cd/m2. So far, there are no indications of superior visual capacities in dim light in Bourke’s parrots compared to budgerigars. In Paper IV, I have studied how colour vision in birds mediates spatial information. Colour patterns function as signals in bird communication, such as the plumage coloration of a courting bird. Much is known about birds’ perception of colour contrast, but the spatial tuning of colour vision in birds has been completely unknown. Here, I present, for the first time, a contrast sensitivity function for isoluminant colour patterns in an animal besides humans, the budgerigar. Paper IV shows that colour vision is used for the detection of colour contrast in larger fields while achromatic vision is used to detect fine detail. Similar finding has been made in humans suggesting that this is a general property of vertebrate vision. With Paper IV, it is now possible to account for both colour contrast and the spatial arrangement of colours in patterns when exploring bird communication.