Radio waves disrupt the magnetic "compass" in robins, according to a study that is likely to fuel debate about the safety of electronic devices.
In a long and careful experiment, German scientists found that migrating robins became disorientated when exposed to electromagnetic fields at levels far lower than the safety threshold for humans.
The frequencies were in the medium-wave band used by AM radio - not the bands used by mobile phones, whose safety has been contested.
"For decades, it has been hotly debated whether man-made electric and magnetic fields affected biological processes, including human health," the authors, from Oldenburg University, wrote in Nature.
The tests show "a reproducible effect of anthropogenic (man-made) electromagnetic noise on the behaviour of an intact vertebrate".
Birds have long been thought to navigate using light and the earth's magnetic field.
Where their magnetic "compass" is and how it works remain unclear - research in homing pigeons suggests it may derive from an iron-rich crystal in their beaks called magnetite.
Seven years ago, the Oldenburg researchers were surprised to find that European robins became confused when they made a stopover on the campus.
Intrigued, the scientists prepared a wooden hut with aluminium sheeting on its walls and "earthed" by a cable to the ground. This virtually eliminated electromagnetic radiation in the range from 50 kilohertz to 20 Megahertz range but had no effect on the earth's magnetic field.
Over seven years, experiments showed that when the screening was in place, birds in the hut adopted their normal position for migration.
But when the screening was removed or the birds exposed once more to a gadget emitting background electromagnetic noise, they were disoriented.