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 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.