True aquatic kingfishers are a generally secretive family (Alcedinidae) with dramatic feeding habits. Most are brightly colored, often solitary fishers. They usually nest in banks along the riverine habitats in which they live. A smaller number of kingfishers are terrestrial birds that are somewhat more social but still strictly carnivorous and usually found near water. Both groups have exceptional vision, especially for their watery niche.
Most aquatic kingfishers hunt by hovering above the water where their prey lives. They wait until the target fish appears in the proper position, and then they drop like a stone to pick the fish out of the water. The hunt is relatively brief, but thoroughly exciting to witness.
Visually, they accomplish tasks that are nothing short of astonishing. Each of their two eyes has two foveae. The fovea is the area of an eye with the best vision because of the concentration of visual cells called photoreceptors. You use your fovea for best vision—for example, you are reading this sentence with your foveae.
Kingfishers have two foveae in each eye, with one fovea near the beak, and having the best vision by virtue of the highest concentration of photoreceptors. This is the fovea a kingfisher will use to sight its prey during the period of hover. During the drop to the water’s surface, the kingfisher sights the fish with this nasal fovea with the sharpest vision. But, once the kingfisher’s beak hits the water, the fish senses the vibration and shock wave coming from that beak entrance. The fish, being alarmed, may respond by trying to escape in an unpredictable manner, and if the kingfisher can’t react to that movement and direction, the hunt will be unsuccessful.
By the time the kingfisher can determine the direction of the target fish, his eyes will be close to or in the water. This changes the angle of the incoming image because of the index of refraction of water.
The kingfisher solves this problem with a second fovea in each eye. Once the eyes are immersed in water, the image of the fish is focused on the second fovea in each eye. That means that there is stereoscopic visualization of the prey as it tries to dart away, an action that is usually not successful.
But, in order to keep the image focused on both foveae, the lens has to be oval and the second fovea has to be in the periphery of the eye at the edge of the retina.
This unusual anatomic variant permits the kingfisher to be virtually unerring in its hunt.
Little work has been done to understand the optics of these eyes, but two images are attached below illustrating the two foveae and the asymmetrical lens. These are taken from an article referenced below. Kolmer VW: Uber das Auge des Eisvogels (Alcedo attis attis). Arch f.d. ges. Physiol., Bd. 204,pp266-274. This is a most interesting arrangement and unlike that used by any other animal on earth.
Oval kingfisher lens in upper image and a cut section of the eye in the lower image.