Simple SummaryBirds often exhibit differences in locomotion, foraging, and predator detection, many of which are often reflected in their eye sizes. Therefore, understanding the coevolutionary relationships between eye size and ecological factors, behaviours and brain size in birds is essential. Our results indicate that species with larger eye sizes reside in dense habitats, feed on invertebrates or vertebrates and have larger brains, suggesting that habitat openness, food type and cognition play critical roles in shaping visual sensitivity and resolution. However, we did not find any correlation between eye size and migration behaviour or foraging habitat, indicating that these factors are likely not major drivers of eye size evolution.AbstractThe eye is the primary sensory organ that obtains information from the ecological environments and specifically bridges the brain with the extra environment. However, the coevolutionary relationships between eye size and ecological factors, behaviours and brain size in birds remain poorly understood. Here, we investigate whether eye size evolution is associated with ecological factors (e.g., habitat openness, food type and foraging habitat), behaviours (e.g., migration and activity pattern) and brain size among 1274 avian species using phylogenetically controlled comparative analyses. Our results indicate that avian eye size is significantly associated with habitat openness, food type and brain size. Species living in dense habitats and consuming animals exhibit larger eye sizes compared to species living in open habitats and consuming plants, respectively. Large-brained birds tend to possess larger eyes. However, migration, foraging habitat and activity pattern were not found to be significantly associated with eye size in birds, except for nocturnal birds having longer axial lengths than diurnal ones. Collectively, our results suggest that avian eye size is primarily influenced by light availability, food need and cognitive ability.