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 been tested previously 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 migrating birds. These data do not support Leuckart's Law: for 88 bird species across 10 orders, a regression of absolute eye axial length versus flight speed explains virtually none of the variance, with an r2 value of 0.001. Regressions of relative eye size versus air speed are significant (P < 0.000, r2=0.454), but with the opposite pattern: faster-flying birds have smaller relative eye volumes than slower-flying birds. Anseriformes, the fastest-flying birds in our sample, exhibit only moderate eye sizes, while the Accipitridae, grouped within the Falconiformes, are the largest-eyed birds measured but have only moderate flight speeds. We, therefore, conclude that Leuckart's Law as a simple expression of flight speed is not useful for explaining bird eye sizes.