The scaling of eye size with body mass in birds

We developed a simple method that uses skulls to estimate the diameter, and hence the mass, of birds? eyes. Allometric analysis demonstrated that, wit hin five orders (parrots, pigeons, petrels, raptors and owls) and across 10 4 families of flying birds, eye mass is proportional to (body mass)(0.68) o ver a range of body masses (6 g-11.3 kg). As expected from their habits and visual ecology, raptors and owls have enlarged eyes, with masses 1.4 and 2 .2 times greater than average birds of the same weight. Taking existing rel ationships for flight speed on body mass, we find that resolution increases close to (flight speed) (1.333). Consequently, large birds resolve objects at a longer time to contact than small birds. Eye radius and skull size co -vary in strict proportion, suggesting common physiological, aerodynamic an d mechanical constraints. Because eye mass scales close to brain mass, meta bolic rate and information processing could also be limiting, but the preci se factors determining the scaling of eye to body have not been identified.