This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Neuroscience. Please check back later for the full article.
Diurnal raptors (birds of the orders Accipitriformes and Falconiformes), renowned for their extraordinarily sharp eyesight, have fascinated humans for centuries. The high visual acuity in raptors is possible due to their unusually large eyes, both in relative and absolute terms, and a very high density of cone photoreceptors. Some large raptors, such as wedge-tailed eagles and the Old World vultures, have visual acuities twice that of humans, and six times that of ostriches, the animals with the largest terrestrial eyes. The highest density of cones occurs in one or two specialized retinal areas, the foveae, where, at least in some species, rods are lacking. The central deep fovea allows for the highest acuity in the lateral visual field that is probably used for detecting prey from a large distance. Actively hunting raptors have a second, shallower, temporal fovea that provides sharp vision in the frontal binocular visual field. Scavenging carrion eaters do not possess a temporal fovea, which might indicate different needs in foraging behavior.
Diurnal raptors, like most birds, have tetrachromatic color vision, based on four spectral types of cones sensitive to violet, blue, green, and red light. However, unlike most birds, their eyes are not very sensitive to ultraviolet light because it is strongly absorbed by the cornea and lens. Four cone types are present in the central fovea; thus, diurnal raptors might possess high-resolution tetrachromatic vision. However, because cones are narrow and densely packed and because rods are absent in the central fovea, the visual acuity of diurnal raptors drops dramatically as light levels decrease. These and other visual properties underpin prey detection and pursuit and reveal the ways in which these birds’ vision is highly tuned to make them successful diurnal predators.