Cells from the inner ear of pigeons stained with a chemical that turns iron bright blue in color. (Photo : Institute of Molecular Pathology)

There are still many pieces to fall in place before scientists complete the puzzle of how each year millions of birds make epic journeys guided by the Earth's magnetic field. But the latest development at Keays lab at the Research Institute of Molecular Pathology in Vienna has added some important pieces to this puzzle, but researchers are still far off from fully understanding how birds use the magnetic field.

According to the latest report, birds have iron balls in sensory neurons. Found in each ear, the cells - called hair cells - are what help birds detect sound and gravity. Each cell has just one iron ball, and the ball is in the same place in every hair cell.

"It's very exciting. We find these iron balls in every bird, whether it's a pigeon or an ostrich," said Mattias Lauwers who, according to a press release, discovered them. Humans do not have iron balls in our sensory neurons, Lauwers said.

The find builds on work done last year by David Keays which refuted the idea that iron-rich cells in the beaks of pigeons were magnetic sensors; they turned out to just be blood cells.

"These [hair] cells are much better candidates, because they're definitely neurons. But we're a long way off to understanding how magnetic sensing works - we still don't know what these mysterious iron balls are doing." said Keays. "Who knows, perhaps they are the elusive magnetoreceptors. ... only time will tell."

The paper "An Iron-Rich Organelle in the Cuticular Plate of Avian Hair Cells" is published online by Current Biology.