Read the full post on Wired UK:
"...Researchers led by University of California, Irvine physicist Thorsten Ritz (.pdf) showed in 2004 that, although robins had no trouble pointing their beaks toward Africa under the influence of Earth's magnetic field alone, adding a second, shifting field destroyed their inner compasses. That second field was so weak -- less than one-third of one per cent of Earth's field -- that it could only have influenced a quantum-sensitive system.
"It shouldn't be the case that the birds would even know that this had happened," Benjamin said. "If someone changed the brightness of the scene that you're seeing by a-third of 1 percent, you would struggle to know that it even happened. It certainly wouldn't muck up your vision...."