What Most birds have exceptionally good vision facilitated through a thick, photoreceptor-packed retina, which is among the most oxygen demanding tissues in the animal kingdom. Yet, the bird retina does not possess an internal capillary blood supply, so the oxygen diffusion distance is over 20 times higher than within the neural tissues in human. This project seeks to identify the alternative physiological mechanism permitting oxygen delivery to the avascular bird retina, and to determine how the evolutionary origin of this mechanism impacted visual performance in the evolutionary history of birds. Why For over a century, evolutionary biologists have worked on the evolution of the complex vertebrate eye as a test case for evolutionary theory. However, next to nothing is known about the necessary nutrient supply mechanisms required to support sensory evolution. This project is important, as it is expected to unravel the respiratory mechanism supporting the highest visual performing eyes amongst vertebrates, and to provide insight into how complex physiological systems evolve over time. How Retinal morphology and mechanisms for retinal oxygen supply will be quantified in birds as well as in reptiles that have less specialized retinae. This information is then used to reconstruct retinal oxygen supply evolution in the line of descent connecting the ancestor of tetrapods and birds in order to identify the most efficient mechanisms for boosted retinal oxygen supply. After having identified the specific components of this respiratory mechanism, each component will be inhibited individually or in combination in birds in vivo while measuring retinal oxygen levels. This physiological-engineering approach allows for experimental resurrection of the possible physiological transition forms of birds' ancestors to provide insight into pathways for evolution of complex systems. SSR Mismatches between oxygen delivery and demands result in rapid dysfunction of neural tissue leading to a range of life-threatening pathologies. Similarly, capillary dysfunction reduces the oxygen supply to in the human retina, which is the leading cause of blindness in the western world. Identifying efficient new mechanisms for oxygen delivery that are less reliant on high capillarity may potentially inform new therapeutic solution for treating a range of ischemic pathologies in humans.