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

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.

The Orientation of Visual Space from the Perspective of Hummingbirds

“…Visual control of the bill tip is likely an important factor that determines binocular vision traits in many bird species (Martin, 2009 , 2017b ; Tyrrell and Fernández-Juricic, 2017a ). Hummingbirds have narrower binocular fields relative to some visually guided foragers (e.g., chickadees, crows, sparrows; Troscianko et al, 2012 ; Moore et al, 2013 , 2015 ), and longer bills relative to their body size.…”

“…The binocular field is important for precise bill control and object inspection at close distances (Martin, 2009 , 2017b ; Tyrrell and Fernández-Juricic, 2017a ). One mechanism to align the bill within the environment may be to use the binocular visual field, and portions of the retina specialized for high acuity that project into the binocular field.…”

Flight morphology and visual obstruction predict collision risk in birds

“…Examples of species with relatively long beaks that are consistently identified as collision-prone include the whitebreasted nuthatch, brown creeper, and gray catbird (Table S7). This general result suggests that long beaks that obstruct the frontal visual field [15] may also impair the ability of small birds to avoid collisions. With impaired vision, a bird might be more apt to misinterpret window reflections as vegetation or open sky, or be more prone to guidance errors caused by artificial light.…”

“…We therefore hypothesized that eye size may influence collision risk, and we predicted that birds with larger eyes, and thus greater visual acuity, may have lower vulnerability scores. Another trait that impacts frontal vision is the beak; long beaks relative to body size can produce a longer anterior blind area, and consequently, a narrower binocular field [15]. This blind area may limit the ability for birds to detect oncoming objects or to gauge distance, particularly when objects lie in the frontal plane.…”

Clinical findings and normative ocular data for free‐living Anna's (Calypte anna) and Black‐chinned (Archilochus alexandri) Hummingbirds

“…[32][33][34] However, increased bill length in visually guided foragers is often related to narrower binocular fields because the tip of a longer bill is more easily encompassed by a smaller binocular field. [35][36][37][38] Overall, the position of the eyes of the hummingbirds may have exemplified a compromise between use of binocularity for foraging and wide visual coverage for detection of predators. Data presented here suggest an ocular disease prevalence in free-living hummingbirds of only 2.28%.…”