Spatial Vision in Birds: Anatomical investigation of spatial resolving power
Year: 2015
Type: dissertation
Abstract: Avian eyes are big both in relative and absolute terms, thus the importance of vision to birds is obvious. Even though the general eye plan is rather conservative throughout the group, there is a great variation in visual capabilities. In this thesis I present four studies on different aspects of spatial vision in parrots, procellariiform seabirds and birds of prey. In Paper I we studied retinal ganglion cell topography and anatomical spatial resolution in two Australian parrots, budgerigar and Bourke’s parrot, inhabiting open terrain. Differently than expected, we did not find a horizontal visual streak, an elongated area of increased ganglion cell density, which would be predicted by the “terrain theory”. In addition, we found that anatomical spatial resolution based on ganglion cell density is lower than behaviourally determined visual acuity. In Paper II, we compared spatial resolution and optical sensitivity in two procellariiform seabirds with contrasting nesting and foraging strategies. As predicted, the Leach’s storm-petrel had lower visual acuity than the Northern fulmar, however similar optical sensitivity at the level of single rod photoreceptor. Additionally, both species had a well-pronounced horizontal visual streak that supports the “terrain theory”. In Paper III, we studied the development of the visual and olfactory system in the Leach’s storm-petrel juveniles. Our results indicated, that fine-tuning of retinal ganglion cell topography does not happen early in development, and that the ganglion cell layer continues to mature throughout the nest period. In addition, we found that two-weeks old juveniles lack basic phototactic and optokinetic reflexes. In Paper IV we used transmission electron microscopy and immunohistochemistry to investigate the presence of double cones in the foveae of the red kite, common buzzard, Eurasian sparrowhawk and peregrine falcon. The double cone-free zones in the central fovea differed in size between species. We also found double cone-free zone in the temporal fovea of the common buzzard and peregrine falcon, but not in the Eurasian sparrowhawk. In three species of raptors, in which we studied opsin expression, we found violet and green sensitive cones in the central fovea, and thus assume that raptors have all four types of single cones in the central fovea. These findings indicate, that single cones, not double cones, mediate the high spatial resolution in the raptor fovea.
Author: Mindaugas Mitkus
Language: English
Subfield: Cognitive Neuroscience
Field: Neuroscience
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W1924427479