Electrical devices may disrupt the migration of some birds, a study suggests.
A German team has found that weak electromagnetic fields produced by equipment plugged into mains electricity and AM radio signals interfere with the animals' "internal compass".
They believe the effect is greatest when birds fly over urban areas.
The study is published in the journal Nature.
Prof Henrik Mouritsen, from the University of Oldenburg in Germany, who carried out the research, said: "At first, I was highly sceptical that this could be the explanation.
"But if you have seemingly unlikely effects then the proof needs to be much stronger - and that is why we have done so many experiments over seven years and it has taken a long time before we were confident to come out with this to the public."
Some birds perform remarkable feats of navigation, migrating half way around the world.
It is thought that a built-in magnetic compass, which senses the Earth's magnetic field, helps them to find their way.
Prof Mouritsen told BBC News he stumbled across the fact that low frequency waves could be interfering with this by accident while studying European robins.
"The basic experiment we do in bird navigation research is that we put birds into an orientation cage," he explained.
"They are so eager to migrate, that they will jump in the direction in which they want to fly, and if you turn a static magnetic field in the horizontal plane they will start to jump in a different direction.
"That experiment has worked for more than 40 years in a number of locations.
"But here in Oldenburg, we couldn't get that basic experiment to work until one day we got the idea to screen these huts on the inside with aluminium plates so the electromagnetic noise was reduced about 100 times.
"And suddenly the birds started to orientate."
Over the course of the next seven years, he and his team carried out numerous experiments to look at how the weak electromagnetic field affected the behaviour of the robins.
In essence, he found that birds exposed to electromagnetic "noise" between 50 kHz and 5 MHz lost all sense of direction. But when the field was blocked out, they found their bearings again.
Prof Mouritsen said that migratory birds flying over towns and cities, where there are more homes and businesses that use electrical devices, would be most affected - and they would probably resort to back-up navigational systems.
"The birds wouldn't be completely lost because they have three different compasses: a star compass, a sun compass and a magnetic compass, and they work independently of each other. As long as it is clear they should be fine with their sunset compass or star compass."
Scientists do not yet understand how a bird's magnetic compass works, but there is some evidence that they use the quantum phenomenon of electron spin to navigate.
"A very small perturbation of these electron spins would actually prevent the birds from using their magnetic compass," Prof Mouritsen said.
"The energies (of the electromagnetic field) are so low in intensity that any physicists will tell you they can't have an effect on a process based on conventional physics.
"Given this effect is real, we find it very difficult to come up with an explanation that is not quantum based."
Commenting on the research in the journal Nature, Joe Kirschvink from the California Institute of Technology (Caltech), said that for some time the field of magnetobiology had been viewed as "a stomping ground for charlatans".
He told the BBC: "The fringe of the fringe of this field is radio field sensitivity."
But he said that this study was "convincing" and "rigorous".
"I would now like to see another independent group replicate this and I would like to see why some other groups have found such dramatically different things," he said.
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