A theory that explains the famous navigation skills of homing pigeons has been shot down by scientists.
Experts had thought iron-rich nerve cells in the birds' beaks acted like a natural compass to help them sense magnetic fields.
But a new study shows that the iron-rich cells are in fact macrophages - specialised white blood cells which form part of the immune system.
The cells play a role in recycling iron from red blood cells, but produce no electrical signals that can stimulate neurons.
Study leader Dr David Keays, from the Institute of Molecular Pathology in Vienna, said: "The mystery of how animals detect magnetic fields has just got more mysterious. We had hoped to find magnetic nerve cells, but unexpectedly we found thousands of macrophages, each filled with tiny balls of iron."
The research is published in the journal Nature.
Dr Keays' team used magnetic resonance imaging (MRI) and computed tomography (CT) scans to create a "three-dimensional blueprint" of the pigeon beak.
The scientists mapped the location of iron-rich cells, and found that their distribution and number did not suggest a role in magnetic sensation.
Ultrasound analysis of the cells revealed features normally found in macrophages. The cells also appeared to be associated with a particular immune system molecule.
Cells that act as internal compasses in birds remain undiscovered, the researchers concluded. "We have no idea how big the puzzle is or what the picture looks like, but today we've been able to remove those pieces that just didn't fit," said Dr Keays.