Writing in the journal Current Biology, lead author Katrin Stapput of Goethe-Universitat in Frankfurt says robins have unique molecules in the retinas of their right eyes that become active when struck by blue light.

The presence of magnetic fields affects how the molecules behave after exposure to the blue light, causing shading in the bird’s field of vision that can be used to determine which direction to fly.

“Both the visual and magnetic images involve variations in light and shade, but visual images tend to have sharp lines and edges, while the magnetic images have more gradual changes from light to dark,” the article says.

This shading is superimposed over the robin’s normal vision, but only in the right eye.

Stapput says that if the bird’s right eye is covered, it loses the ability to see the magnetic field, but not if vision in the left eye is blocked.

They made that determination by fitting birds with special goggles that obscured the vision of different eyes in various test foul.

Those with the right eye covered appeared to be disoriented and headed in random directions, while those with the left eye covered set off in the expected migratory direction.

Photos: Goethe-Universitat and Wikimedia Commons