Seeing Danger and Opportunity: Visual Specializations in Avian Predators and Prey

Predators and prey are locked in a sensory arms race to detect each other as early as possible and to quickly gather the information needed to make life or death decisions. It is well known that birds have excellent visual systems, but it is not known which components of visual information are important in predator-prey encounters. Here, I focus on the role of vision in predator-prey encounters and the visual specializations that different species have evolved to thrive as predators, as prey, or both. My research shows that predators with a greater need to visually control their own beak are actually able to see more of their own beak with binocular vision. In fact, I show more generally that variation in avian binocular vision is driven, in part, by a blind spot in front of the head rather than binocular information itself. This dissertation also discusses visual specializations of passerine songbirds, which often play the roles of predator and of prey. For example, eastern meadowlarks (Sturnella magna) have an unusual retinal morphology that allows them to spot aerial predators above the horizon in their expansive open habitats. Eastern meadowlarks are also able to see much of their own beak, which allows them to visually control their beak while they expose and depredate arthropods underneath the soil surface. European starlings are able to use large eye movements to visually guide a similar foraging tactic, but the large eye movements can also be made in reverse to look above and behind their head for predators. Small passerines that catch prey on the wing have evolved multiple strategies for predatory efficiency. Tree swallow vision is quite different than that of other passerines, but remarkably similar to the visual systems of raptors. Tree swallows and raptors both have two foveae per eye, elongated eyes, small binocular fields, and posterior blind areas. But because tree swallows are also prey, they do have a reduced blind area relative to raptors. Like tree swallows, Empidonax flycatchers are obligate predators, but they have evolved a very different visual system. The flycatcher eye is not elongated and only has one fovea, but flycatchers have evolved an entirely new photoreceptor that has electron-dense megamitochondria surrounded by orange oil droplets. The new photoreceptor may allow flycatchers to view prey in ‘slow motion’. Overall, there are conserved aspects of predator-prey encounters that lead to convergence on similar visual configurations (e.g., swallows, falcons, hawks). Yet minor differences in how an animal hunts can fundamentally change the building blocks of vision (e.g., flycatchers, swallows)