A comparative study of deep avian foveas

The foveas of nine avian species, initially selected for the presence of a deep fovea and representing a wide range of eye sizes and ecological habits, were studied with quantitative light microscope techniques. Considerable variation was observed in the location and configuration of the avian foveas, although they appeared to be 'convexiclivate' in shape when compared with the fovea of the rhesus monkey. Comparisons of foveal cell densities (receptor nuclei and ganglion cells) across species showed an increase in the average number of cells/visual degree2 with increasing eye size; similarly, an increase occurred in receptor nuclei relative to ganglion cell density. Thus, smaller eyes showed a coarser retinal grain and a lower 'concidence ratio' of receptors to ganglion cells than was found in the largest eyes. There appeared to be no relationship between receptor densities/mm2 and (a) eye size, (b) depth of foveal clivus, or (c) width of foveal clivus. However, a negative correlation was generally observed between the width of the foveal clivus and eye size. Two foveas were seen in the red-tailed hawk, goshawk, sparrow hawk, and least tern. The central fovea was more differentiated, with greate densities of both receptor nuclei and ganglion cells than was observed in the emporal fovea of the same species. Further conclusions, particularly with respect to potential visual acuity, await quantitative measurements of foveal cone densities across species.