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?