How do pigeons navigate? It's not the tiny iron balls in their ears.

"It's very strange to just have a ball of iron sitting in there."

This is an Inside Science story.

Many animals can sense things that humans cannot -- the humble pigeon, for instance, is capable of sensing and navigating via the Earth's invisible geomagnetic field. Scientists have long been interested in how this is possible. Now, a new study published in the journal PNAS suggests that promising structures in the pigeons' inner ears are not, in fact, the answer.

A few different parts in the pigeon's anatomy have been suggested as sources of magnetoreception, including structures in the eye and in the region of the pigeon's beak. But the inner ear has also been a candidate, due to the presence of iron-rich, roughly ball-shaped objects called cuticulosomes within sensory cells of the ear.

In this new study, scientists in Australia and Europe investigated these structures by taking thin slices of tissue from a pigeon's inner ear and laying them on top of flat diamond chips. The chips fluoresce, and this fluorescence can be intensified or dimmed by nearby magnetic substances such as the cuticulosomes. Using a camera and a microscope to measure these changes, the scientists were able to estimate how sensitive the cuticulosomes would be to the Earth's magnetic field.