Visual fields and their functions in birds

Abstract

Among birds there are considerable interspecific differences in all aspects of visual fields. However, it is hypothesised that the topography of the frontal binocular portion of fields are of only three main types, and their principal functions lie in the degree to which vision is used in the guidance of the bill (or feet) towards food objects or for the provisioning of chicks. In the majority of birds, the width of the frontal binocular field is narrow (20°–30° maximum). It shows a high degree of similarity across species and appears to be independent of phylogeny or ecology. Binocularity appears not to be concerned with higher level visual processing involving the combination of information from the two eyes (as in, for example, stereoscopic vision). Binocularity is concerned with gaining independently, in each eye, information which is derived from the symmetrically expanding optic flow-field, which specifies the direction of travel of the head and its time to contact an object, as in pecking or lunging at food items. Species which do not provision their chicks, and whose foraging is guided by tactile cues or which filter feed, have much smaller binocular overlap (10°) and this seems sufficient to control flight. These birds gain comprehensive visual coverage of the celestial hemisphere and show reduced vigilance behaviour. The visual fields of owls, which combine more extensive binocular overlap (50°) with a large blind area behind the head, may not be primarily associated with nocturnal activity. Visual fields of this type are not found in other nocturnally active birds such as Oilbirds, nightjars and kiwis. The type of visual field found in owls may be a result of large eyes combined with elaborate outer ear structures that are placed within a relatively small skull. Eye movements of significant amplitude do not occur in all birds. However, eye movements of between 14° and 18° occur in species such as herons, hornbills and cormorants and can result in the spontaneous abolition of binocularity. These eye movements are non-conjugate and can produce markedly asymmetric visual fields. The width of any blind area above the head is a function of eye size, with the largest eyes associated with optical adnexa, (eye lashes, brows). These may be associated with avoiding imaging the sun on the retina. However, many small-eyed birds have no optical adnexa and cannot avoid seeing the sun.

Appendices

Appendix 1

The ophthalmoscopic technique for determining retinal visual fields

This technique has been used in a range of birds of different phylogeny, ecology and feeding techniques and readily permits interspecific comparisons (Martin and Coetzee 2004; Martin et al. 2005, 2004b; Martin and Prince 2001). For a detailed description of the apparatus and methods see Martin and Katzir (1994a). Briefly, each bird is held in a foam rubber cradle with its head held in position at the centre of a visual perimeter by a holder specially manufactured to hold the bill of each species. The perimeter’s co-ordinate system follows conventional latitude and longitude with the equator aligned vertically in the birds’ median sagittal plane and this co-ordinate system is used for the presentation of visual field data (Fig. 2). The bill is closed and the head positioned so that the tip of the lower mandible projects in the approximate direction adopted spontaneously by the bird when at rest. Head position is also recorded in photographs of birds held in the hand in the open or when at rest in the field. The results are corrected to this spontaneously adopted head position.

The eyes are examined using an ophthalmoscope mounted on the perimeter arm. For each eye, the visual projections of the following are usually determined as a function of elevation in the median sagittal plane at 10° intervals: (1) the limits of the retinal visual field, and (2) the edges of the pecten. From these data (corrected for viewing from an hypothetical viewing point placed at infinity) a topographical map of the visual field and its principal features is constructed. These features are: (1) the monocular fields, the visual field of a single eye; (2) the binocular field, the area where monocular fields overlap; (3) the cyclopean field, the total visual field produced by the combination of both monocular fields; and (4) projection of the pectens. Each pecten creates a blind area within each eye’s visual field. The pecten is a highly vascular and pigmented structure that overlies the exit of the optic nerve and functions as a nutrient organ within the anterior chamber of the eye. Its projection provides a landmark in the field of each eye. Depending upon the species and the time available to hold the birds constranied in this way, it is possible to measure limits of almost the entire frontal visual field from directly below the head through approximetly 270° in the sagittal plane to the horizontal behind the head. Determining the positions of the visual field margins both directly in front and behind the head, i.e. in an approximately horizontal plane, allows determination of monocular field widths and the extent of the cyclopean field in this plane. In some birds, spontaeous eye movements are readily observed and their amplitude as a function of elevation are determined by making light tapping sounds or flashing a small light source in the periphery of the visual field, and then determining the maximum and minimum limits of the retinal margin at each elevation. These procedures are non-invasive and follow guidelines established by the United Kingdom, Animals (Scientific Procedures) Act, 1986.

Appendix 2

Bird species in which visual fields have been determined using the ophthalmoscopic reflex method (14 Orders; 20 families; 32 spp.)