Robins lost their sense of direction because of a university's electrical noise
When Henrik Mouritsen tested caged robins on his campus, they could not orient at all. Shielding their huts with earthed aluminium fixed it; removing the earthing broke it again. Faint radio interference was scrambling a compass that seems to run on quantum physics inside the birds' eyes.
Many animals can sense Earth's magnetic field, from arthropods and molluscs to fish, amphibians, reptiles, birds and mammals. For most of the twentieth century the proof was behavioural only. In 1972 Roswitha and Wolfgang Wiltschko showed migratory birds respond to the field's direction and its dip angle, but how they did it remained unclear. Claims of iron-based magnetite receptors in bird beaks have repeatedly collapsed; cells reported in pigeons in 2003 turned out to be macrophages.
The leading explanation is the radical pair mechanism. Two molecules with unpaired electrons flip rapidly between aligned and opposed spin states, and that flipping is exquisitely sensitive to magnetism. Since Earth's field is only about 0.5 gauss, this is the one credible route by which it could alter chemistry. In 2000 researchers proposed cryptochrome, a protein in the eye's rod cells, as the molecule responsible: blue light kicks an electron free and creates an entangled radical pair. One form, Cry4a, peaks in migrating birds during spring and autumn, and the robin version is far more magnetically sensitive than the equivalents in non-migratory pigeons and chickens.
Behaviour fits the quantum model. In 2004 Thorsten Ritz disrupted birds' orientation with a weak radio signal tuned to the cryptochrome oscillation, which would not affect an iron compass. Birds also fail to notice when the field is flipped 180 degrees, something a magnetised needle would register instantly. Mouritsen's campus accident, begun from 2007 as an attempt to repeat Ritz's work, pointed the same way.
Other animals may use other tricks. Magnetotactic bacteria contain magnetite crystals and are simply swung into line, not sensing anything. Sharks and rays may detect the field by induction through their electrosensing ampullae of Lorenzini, and the yellow stingray can tell field strength from inclination in the lab. In a 1991 maze test, 80 percent of giant sea slugs turned toward magnetic east.
Source: Magnetoreception
