Interspecific differences in the visual system and scanning behavior of three forest passerines that form heterospecific flocks
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.