Seabirds & Waterfowl - Sounds Wild
Download Episode: Kingfisher Eyes (MP3 file 1,406 kB)
A kingfisher is perched on a branch over the water, watching a school of salmon fry. When the moment is right, she dives, plunging under the water and emerging with a wriggling fish.
It's quite a feat for a kingfisher to catch a fish underwater because from the air, the fish isn't where it appears to be. Water bends light rays more than air does and viewed from the surface, the fish is actually a few inches away from where it seems to be. Kingfishers solve this with a special feature in their eyes that we don't have. In the back of a human eye, on the retina, is a little pit called the fovea with a lot of visual cells that create an area of very acute vision. The kingfisher has not one but two foveas, separated by a little distance, and their vision shifts smoothly from one to the other as they enter the water. When the kingfisher dives into the water, the second fovea is engaged, and that gives the diving bird high visual acuity at the all-important last moment before it grabs its prey with its bill.
A South American fish called anableps has a similar, but opposite arrangement in its eyes. The fish swims at the water surface and can see above and below the water at the same time. The upper half of its eye is adapted for seeing in the air and the lower half of the eye is adapted for seeing under water.