Spatial Vision in Birds: Anatomical investigation of spatial resolving power

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.