Avian binocular vision: It's not just about what birds can see, it's also about what they can't.

Luke P Tyrrell,Esteban Fernández-Juricic

doi:10.1371/journal.pone.0173235

Abstract

With the exception of primates, most vertebrates have laterally placed eyes. Binocular vision in vertebrates has been implicated in several functions, including depth perception, contrast discrimination, etc. However, the blind area in front of the head that is proximal to the binocular visual field is often neglected. This anterior blind area is important when discussing the evolution of binocular vision because its relative length is inversely correlated with the width of the binocular field. Therefore, species with wider binocular fields also have shorter anterior blind areas and objects along the mid-sagittal plane can be imaged at closer distances. Additionally, the anterior blind area is of functional significance for birds because the beak falls within this blind area. We tested for the first time some specific predictions about the functional role of the anterior blind area in birds controlling for phylogenetic effects. We used published data on visual field configuration in 40 species of birds and measured beak and skull parameters from museum specimens. We found that birds with proportionally longer beaks have longer anterior blind areas and thus narrower binocular fields. This result suggests that the anterior blind area and beak visibility do play a role in shaping binocular fields, and that binocular field width is not solely determined by the need for stereoscopic vision. In visually guided foragers, the ability to see the beak—and how much of the beak can be seen—varies predictably with foraging habits. For example, fish- and insect-eating specialists can see more of their own beak than birds eating immobile food can. But in non-visually guided foragers, there is no consistent relationship between the beak and anterior blind area. We discuss different strategies—wide binocular fields, large eye movements, and long beaks—that minimize the potential negative effects of the anterior blind area. Overall, we argue that there is more to avian binocularity than meets the eye.

Highlights

Despite most vertebrates having their eyes positioned laterally rather than frontally, the discussion about binocular vision in vertebrates has traditionally revolved around the extraction of relative depth information from stereopsis [1,2,3]
The fact that depth information is available in the absence of stereopsis [11], coupled with the existence of binocularity in species that likely lack stereopsis [12], certainly leaves room for the idea that binocular vision is about much more than stereopsis and depth perception
Considering all bird species studied and controlling for beak length (Table 1), we found that as binocular field width increases, the length of the anterior blind area became shorter (Table 1; Fig 3A)

Summary

Introduction

Despite most vertebrates having their eyes positioned laterally rather than frontally, the discussion about binocular vision in vertebrates has traditionally revolved around the extraction of relative depth information from stereopsis [1,2,3]. Multiple factors, including disconjugate eye movements [7,8], seem to suggest that stereopsis is not present in most bird species despite the presence of a binocular overlap [9,10]. The fact that depth information is available in the absence of stereopsis [11], coupled with the existence of binocularity in species that likely lack stereopsis [12], certainly leaves room for the idea that binocular vision is about much more than stereopsis and depth perception

Methods

Results

Conclusion