Video of a kingfisher bird hunting, shot at 600 frames/second (FPS). Kingfishers adjust for water refraction by using two visual focal points (foveae) in each eye to accurately calculate the prey's true depth and position before diving. They can strike at speeds up to 40 kilometers/hour.
When light transitions from water to air, it bends away from the normal line at the boundary due to the differing refractive indices. Because of this bending, the fish appears higher in the water column than its actual physical location. The kingfisher's brain automatically processes this angular disparity using its dual foveae. This allows the bird to aim its dive trajectory below the visual image of the prey to hit the actual physical target.
Kingfishers possess specialized eyes containing both a central and a temporal fovea. The central fovea provides sharp vision while navigating through the air, while the temporal fovea engages to provide high-acuity, stereoscopic binocular vision under the water. Their eyes have a special membrane which shields them from high-speed impact pressure while acting as a clear underwater lens.
Also, the physical structure of a kingfisher's eye is slightly flattened. When the bird hits the water, the change in pressure and medium causes a shift in the eye's focus. This unique shape, paired with an highly flexible lens, allows them to compensate for the rapid transition from air refraction to water refraction instantaneously.
The song in this video is "Levitation" by Aaron Hibell & Felsmann + Tiley.
