Hawks

Eyesight

Hawks, like most birds, are tetrachromats with four types of colour receptors in the eye. Unlike some birds, but similar to other diurnal raptors, most hawk species are violet-sensitive but cannot perceive ultraviolet light. Hawks also have relatively high visual acuity – the distance at which they can resolve an image – with red-tailed hawks reported to have 16.8 cycles per degree. This is due to the large number of photoreceptors in the retina (up to 1,000,000 per square mm in Buteo, compared to 200,000 in humans), a high number of nerves connecting these receptors to the brain, and an indented fovea, which magnifies the central portion of the visual field. Hawks additionally have two foveae (depressions in the retina with high photoreceptor density) in each eye, one centrally-placed and the other temporally-placed. Finally, the eyeball is elongated, placing the lens far from the retina and giving a long focal length.