Life History and Sociality Predict Variation in Eye Size across Birds

Simple Summary The eye in birds has evolved through time to facilitate foraging and detect potential threats, such as predators. The size of the eye affects how much light is captured and image resolution. Eye size varies across species, and this variation is typically associated with the availability of light in the environment and the foraging needs of each species. Larger eyes could also allow species to detect distant predators more easily, but this has not been tested across species. I predicted that species that are averse to risk and those exposed to high predation risk benefit from larger eyes to enhance the detection of predators. I performed an analysis using 660 different species of birds with various foraging needs and life histories. Controlling for relatedness among species and known correlates of eye size in birds, I found that species that emphasize survival over reproduction, which are expected to be risk-averse, have larger eyes relative to their body size. In addition, species that live alone and cannot rely on others in their groups to decrease the risk of predation also have larger eyes. The results show that the risk of predation through its association with life history and sociality can shape variations in eye size across species of birds.Abstract Over evolutionary times, the eye has acquired several adaptations to improve feeding efficiency and reduce predation pressure. Eye size, in particular, represents a target for selection, as it affects light capture and image resolution. Previous research on variation in eye size across species has focused on ecological factors related to light availability and foraging needs. Larger eyes are also thought to allow species to detect distant predators more easily, but this conjecture has not been examined across species. I predicted that risk-averse species or those exposed to high predation risk benefit from relatively larger eyes to enhance predator detection. To test the prediction, I performed a comparative analysis involving 660 species of birds while controlling for phylogeny, body size and other known ecological correlates of eye size. The results show that species at the slow end of the life history continuum, which emphasize survival over reproduction and are expected to be risk-averse, have evolved relatively larger eyes. In addition, solitary species, which cannot rely on others in their groups to decrease predation risk, are also characterized by relatively larger eyes. The results indicate that predation risk, through its association with life history and sociality, is an important ecological factor in the evolution of eye size across species.