Visual systems and vigilance behaviour of two ground-foraging avian prey species: white-crowned sparrows and California towhees

Abstract

Predator–prey interactions are regulated by the ability of individuals to detect, and then approach or avoid, each other. In visually guided organisms, the prevalent view is that predators have large binocular visual fields and high acuity, whereas prey have wide lateral areas and low acuity, which could affect vigilance behaviour. We characterized the configuration of the visual system (visual fields, retinal topography, visual acuity) and vigilance behaviour (head movement rate) of two ground-foraging avian prey (white-crowned sparrow, Zonotrichia leucophrys, California towhee, Pipilo crissalis) with laterally placed eyes. We found that the binocular field of both species (45°) was actually wider than those of some of their avian predators. Both species also had a single retinal specialization (high ganglion cell density area) located in the centro-temporal sector of the retina, which projected into the lateral and frontal part of the head. Wide binocular fields may increase binocular contrast to detect and visually guide the bill towards prey items. Both species had wider lateral visual fields and faster head movement rates than some of their predators, probably to enhance detection and visual tracking of predators. California towhees made faster sideways movements of the head than did white-crowned sparrows, probably to cover visual space more quickly with their retinal specialization because of the comparatively lower spatial resolution of their retinal periphery. Alternatively, California towhees might move their heads more rapidly to monitor for potential risks (e.g. competitors, predators), as they rely mostly on personal information because of their degree of territoriality. Our findings suggest that the visual system and vigilance behaviour of these two avian prey species combine traits to enhance predator detection through large visual coverage and fast head movements, but also to enhance food detection at close range through enhanced binocular vision.