What Makes Bird Vision So Cool
They use their right and left eyes differently. Some may sleep on the wing. They may even be able to see the earth's magnetic field. Welcome to the amazing world of bird vision.
The second circumstance in which it is extremely useful for birds to keep an eye open is when they sleep on the wing--that is, while flying. The idea that they might sleep and fly simultaneously once seemed ludicrous. But ornithologist David Lack and others noticed European swifts ascending into the sky at dusk and not returning until the following morning, and inferred that they must sleep on the wing. More convincingly, a French airman on a special nocturnal operation during World War I reported that as he glided down across enemy lines with his engine off, at an altitude of around 10,000 feet, "We suddenly found ourselves among a strange flight of birds which seemed to be motionless . . . they were widely scattered and only a few yards below the aircraft showing up against a white sea of cloud underneath." Remarkably, at least one was caught and identified as a swift. Of course, neither Lack nor the French airman noticed whether their sleeping swifts had one eye open, but it is a possibility. Glaucous-winged gulls in North America, however, have been seen flying to their roosts with only one eye open, suggesting that they are already sleeping before even reaching the roost.
Examining vision alone makes sense for convenience and clarity, but in reality, of course, birds use their senses in combination. Understanding how they work together increases our understanding of the way birds perceive the world. Take how migrating birds find their way. Biologists have long known that they use the sun and stars to navigate. More recently they have discovered that birds possess an internal magnetic compass.
It's possible that a chemical mechanism based in the eye provides the compass, allowing them to "see" the earth's magnetic fields, while magnetite (a magnetic mineral) receptors in the beak provide the map. The compass may detect the direction of the magnetic field, while the map detects the strength of the magnetic field, and by integrating both types of information the birds can find their way home, whether across a featureless ocean or a large land mass. At the present time we have a good basic understanding of at least some of the senses of birds, but the best is yet to come.
This story originally ran in the May-June issue as "Bird's-Eye View."