By Dr. Becker
Now here’s a word you may never come across in your lifetime (unless you enter a lot of spelling bees): Magnetoception (also called magnetoreception). Do you know what it refers to? It describes the ability to detect a magnetic field to identify direction, altitude or location.
Magnetoception is a skill only a handful of creatures seem to have, including bacteria, some invertebrates (fruit flies, honeybees, and lobsters), homing pigeons, domestic hens, certain mammals, turtles, sharks and stingrays. Humans may or may not possess the ability, depending on who you ask.
Robins Not Only Sense, But Actually SEE, Magnetic Fields… But in Only One Eye
Some birds can sense the Earth’s magnetic field and orient themselves accordingly. As you can imagine, this is a huge benefit for the “frequent flyers” of the avian world, migrating birds.
But one type of bird in particular, the robin, can actually see magnetic fields thanks to a special molecule called a cryptochrome in the retina. The fields appear as patterns of light, shade or color superimposed onto what the birds normally see.
Scientists have learned that in robins, the magnetoception ability is dependent on good vision in the right eye. If the right eye is covered, the birds become disoriented when they fly, but if the left eye is covered, they navigate without a problem. This means the robin’s vision in the right eye acts as a doorway for its magnetic sense. If there is darkness or cloudiness in the right eye, the door stays shut, but light in that eye opens the door and activates the bird’s internal compass.
The guidance mechanism seems to work in such a way that the magnetic field-generated patterns of light, shade or color overlaying what a robin normally sees change as the bird turns and tilts its head during flight. This provides a visual compass composed of contrasting shades of light. But the compass doesn’t depend solely on light – the birds must also have a clear image with their right eye in order to accurately navigate. Their magnetic sense is only a transparent overlay to the images their normal vision provides. If that vision is impaired in any way, the magnetic sense is of no use. (Imagine trying to follow your car’s GPS navigation instructions with a couple inches of heavy wet snow covering your windshield.)
Experts believe robins probably require a clear, focused image to distinguish between inputs from their visual and magnetic senses. Since the magnetic field lies on top of what is seen through normal vision, and both incorporate differences in light and shade, it would seem the birds could become easily confused. However, the images the birds see through normal vision tend to have sharp transitions between light and shade, whereas changes in the patterns superimposed by magnetic fields are smoother and more gradual. Birds are probably aware that sharp changes in contrast are due to the boundaries of actual objects, while more subtle changes are the result of magnetic effects.
Baby Robins Possess a Magnetic Compass in Both Eyes, But Lose the Left One as They Mature
While adult robins have a magnetic compass in their right eye only, as babies, they had a compass in each eye. It seems they lose the left one as they mature.
The change from both eyes to just the right eye occurs gradually. In robins that are no longer babies but not fully grown, the magnetic compass in the left eye can be revived for a time by covering the right eye. According to scientists, this means the eyes themselves aren’t changing. Instead, the brain begins processing input from the eyes in different ways.
A near equivalent in humans is right- or left-handedness. The hand we ultimately prefer as adults only becomes dependably dominant around the age of four or five.