Bird's Eye View

It has long been a puzzle how birds manage to navigate the world without the specialized instruments that humans use—including the human brain. Birds, after all, have only bird brains.

New evidence strongly suggests that birds actually use their visual circuitry to detect the earth's magnetic field and navigate by it. That is, in some sense birds see the earth's magnetic field, even in the dark.

How does it work? A certain chemical found in the eyes of migratory birds responds to electromagnetism, and according to the German team who pursues this research:

"When light hits these molecules, their chemistry changes and magnetism can influence them," Mouritsen said. The molecules might then affect light-sensing cells in the retina to create images, which would help the brain navigate during flight, he added.

It's difficult to imagine what this would look like, since humans are confined to... well, the "visible" parts of the spectrum. One scientist suggested the analogy that perhaps north looks like a dark spot, and all other directions are within some gradient of light. More details here.

How many other senses are possible, unknown to us? Unimagined by us?

Comments

[Usha]

It turns out that birds do actually see the magnetic field. Details are reported over at Not Exactly Rocket Science:

"When cryptochrome is struck by blue light, it shifts into an active state where it has an unpaired electron – these particles normally waltz in pairs but here, they dance solo. The same thing happens in a companion molecule called FAD. Together, cryptochrome and FAD, both with unpaired electrons, are known as a “radical pair”. Magnetic fields act upon the unpaired electrons and govern how long it takes for the radical pair to revert back to their normal, inactive state. And because cryptochrome affects the sensitivity of a bird’s retina, so do magnetic fields.

The upshot is that magnetic fields put up a filter of light or dark patches over what a bird normally sees. These patches change as the bird turns and tilts its head, providing it with a visual compass made out of contrasting shades."

There's an interesting illustration accompanying the article that sheds much light on the phenomenon.