What is binocular vision for? A birds' eye view

Abstract

It is proposed that with the possible exception of owls, binocularity in birds does not have a higher order function that results in the perception of solidity and relative depth. Rather, binocularity is a consequence of the requirement of having a portion of the visual field that looks in the direction of travel; hence, each eye must have a contralateral projection that gives rise to binocularity. This contralateral projection is necessary to gain a symmetrically expanding optic flow-field about the bill. This specifies direction of travel and time to contact a target during feeding or when provisioning chicks. In birds that do not need such control of their bill, binocular field widths are very small, suggesting that binocular vision plays only a minor role in the control of locomotion. In the majority of birds, the function of binocularity would seem to lie in what each eye does independently rather than in what the two eyes might be able to do together. The wider binocular fields of owls may be the product of an interaction between enlarged eyes and enlarged outer ears, which may simply prevent more lateral placement of the eyes.

Extreme binocular vision and a straight bill facilitate tool use in New Caledonian crows

“…Further behavioural testing would be necessary to establish whether NC crows are able to utilize their large binocular fields for stereoscopic depth perception, which may be useful for gauging distances during tool deployment 11 .…”

Vision, touch and object manipulation in Senegal parrots Poicephalus senegalus

“…This probably prohibits the rapid and accurate control of the bill towards objects, which is achieved by birds that peck or lunge for their food [14]. However, like visually guided foragers, Senegal parrots do have a relatively broad frontal binocular field, which could aid inspection of objects held up in the foot.…”

“…'footedness' in grasping objects) [45 -50]. Highest acuity in all avian taxa is thought to occur in the lateral field [14].…”

“…It has been proposed that avian visual field configuration is primarily determined by foraging technique [13,14]. Birds that peck or lunge for their food (e.g.…”

Quasi-Parallax for Nearly Parallel Frontal Eyes

“…Martin (2007Martin ( , 2009) argued that stereopsis is too slow, especially for the purpose of locomotion. Davies and Green (1994)have pointed out that stereopsis involves considerable neural processing and is too slow to control the estimation of distance and depth when a bird is landing upon a perch.…”