Sun shades and eye size in birds

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 dif fer in their phylogeny and ecology. These species can be divided into two g roups. Smaller-eyed sun-observers (axial length <less than>18 mm) have comp rehensive or near comprehensive visual coverage of the celestial hemisphere and are thus unable to avoid viewing the sun, Larger-eyed sun-avoiders (ax ial length >18 mm) have restricted visual fields and various types of optic al adnexa (enlarged brews, hair like feathers on the eye lids and around th e eye) which can prevent solar illumination of the cornea. We suggest that these differences relate to visual rather than pathological problems and ar gue that the reduction of disability glare, produced by sunlight falling di rectly upon the eye, becomes increasingly significant as eye size increases . We propose that the reduced visual fields and optical adnexa of the large r-eyed birds are primarily concerned with the maintenance of high spatial r esolution. Copyright (C) 2001 S. Karger AG, Basel.