Eye size, flight speed, and Leuckart's Law in birds

Birds have the largest eyes, both relatively and absolutely, of any of the terrestrial vertebrates. Large avian eye size has been hypothesized to be an adaptation to flight as part of Leuckart's Law, the idea in biology that more swiftly moving animals have larger eyes. Increased spatial resolution is one result of larger eye sizes and may possibly improve an animal's ability to judge distances, of obvious importance for flight. Leuckart's Law in birds has previously been tested utilizing Plasticine eye models and body mass as a surrogate for flight speed. In this study, we test Leuckart's Law using axial length measurements of eyeballs obtained from wet bird specimens and flight speeds obtained from flying birds. We find no evidence to support Leuckart's Law: for 98 bird species across 6 orders, a regression of eye axial length vs. flight speed explains virtually none of the variance, with an r 2 value of 0.001. Ducks, the fastest‐flying birds in our sample, exhibit only moderate eye sizes, while falcons, the largest‐eyed birds measured, have only moderate flight speeds. We therefore conclude that Leuckart's Law is not useful for explaining bird eye sizes.