Scientists have long known that the birds navigate using the earth's magnetic field. Now, a new study has found subtle mechanics in the brain of pigeons that allow them to find their way.
A team at Baylor College of Medicine in the US identified a group of 53 cells in a pigeon's brain that record detailed information on the Earth's magnetic field, a kind of internal global positioning system (GPS).
However, the study, published in journal Science, leaves open the question of how these "GPS neurons" actually help the birds sense the magnetic field.
"People had reported in the past, establishing that birds do not seem to respond to the polarity of the magnetic field, yet here we have neurons that are in fact doing that," study author Prof David Dickman said.
"That's one of the beautiful aspects of what we have identified, because it shows how single brain cells can record multiple properties or complex qualities in a simple way," he told BBC News.
For their study, Prof Dickman and his colleague Le-Qing Wu set up an experiment in which pigeons were held in a dark room and used a 3D coil system to cancel out the planet's natural geomagnetic field and generate a tunable, artificial magnetic field inside the room.
While they adjusted the elevation angles and magnitude of their artificial magnetic field, they simultaneously recorded the activity of the 53 neurons in the pigeons' brain which had already been identified as candidates for such sensors.
So, they measured the electrical signals from each one as the field was changed and found that every neuron had its own characteristic response to the magnetic field, each giving a sort of 3D compass reading along the familiar north-south directions as well as pointing directly upward or downward.
In life, this could help the bird determine not only its heading just as a compass does, but would also reveal its approximate position, the researchers said.
Each cell also showed a sensitivity to field strength, with the maximum sensitivity corresponding to the strength of the Earth's natural field, they added.
And like a compass, the neurons had opposite responses to different field "polarity", the magnetic north and south of a field, that surprised the researchers most of all.
Several hypotheses hold that birds' magnetic navigation arises in cells that contain tiny chunks of metal in their noses or beaks, or possibly in an inner ear organ.
However, the most widely held among them was thrown into question when researchers found that purported compass cells in pigeon beaks were in fact a type of white blood cell.
Another theory suggests that a magnetic sense may come about in receptors in birds' eyes. When exposed to light, the theory says, molecules called cryptochromes undergo a fleeting change in their atomic makeup whose length depends on their alignment with a field.
The new research throws this latter possibility also into question, as it would work equally well with a north- or south-pointing field.
"We're leaning toward a third receptor in the inner ear, and we're doing experiments to try to determine whether it is in fact a receptor or not," said Prof Dickman.
It's now believed that more than one mechanism may be at work in bird navigation -- in their eyes, beaks or ears -- and Prof Dickman said he is looking forward to getting to the bottom of it.