Sun Shades and Eye Size in Birds

Abstract

Visual field width above the head is significantly correlated (r_s = 0.92, n = 11, p < 0.001) with eye size in a sample of terrestrial birds that differ in their phylogeny and ecology. These species can be divided into two groups. Smaller-eyed sun-observers (axial length <18 mm) have comprehensive or near comprehensive visual coverage of the celestial hemisphere and are thus unable to avoid viewing the sun. Larger-eyed sun-avoiders (axial length >18 mm) have restricted visual fields and various types of optical adnexa (enlarged brows, hair like feathers on the eye lids and around the eye) which can prevent solar illumination of the cornea. We suggest that these differences relate to visual rather than pathological problems and argue that the reduction of disability glare, produced by sunlight falling directly upon the eye, becomes increasingly significant as eye size increases. We propose that the reduced visual fields and optical adnexa of the larger-eyed birds are primarily concerned with the maintenance of high spatial resolution.

Vision in avian emberizid foragers: maximizing both binocular vision and fronto-lateral visual acuity

“…We also found that some proposed associations between visual traits were not as strong in emberizid sparrows as in nonPasseriformes. For example, we did not find a relationship between eye size and blind area width, as predicted by the glare hypothesis (Martin and Katzir, 2000). This could be related to our low sample size (i.e.…”

“…glare effects; Koch, 1989). Species with larger eyes can be more prone to glare effects because of larger optical apertures leading to a greater influx of sunlight (Martin and Katzir, 2000). Positioning the sun's image in any blind spot (i.e.…”

“…Positioning the sun's image in any blind spot (i.e. blind area, pecten) would reduce glare effects, which leads to two alternative solutions for species with larger eyes: larger blind areas (Martin and Katzir, 2000) and/or larger pectens (Fernández-Juricic and Tran, 2007;van den Hout and Martin, 2011). We thus predicted a positive association between eye size and pecten size, as well as eye size and blind area width.…”

Visual perception and social foraging in birds

“…ostriches, eagles or albatrosses) have developed sunshade structures (e.g. eyebrows or eye lash-type feathering) and larger blind areas to minimize sunlight glare [49]. Large blind areas reduce the cyclopean field, which might be affordable in large species owing to their lower risk for predation.…”

“…Large blind areas reduce the cyclopean field, which might be affordable in large species owing to their lower risk for predation. However, species with small eyes generally have smaller blind areas and larger cyclopean fields, because they are not so strongly affected by sunlight and need a wider visual field with which to detect predators [49]. Thus, differences in the size of visual fields seem to be determined by tradeoffs between high acuity, predator vulnerability and sensitivity to light.…”