Together, those results sketched a chain from a changing magnetic field to electrical signals to brain activity — but they left a harder question open: can the signal actually convey direction?

Can the signal keep up?

Kattnig modeled one canal as a small conductive ring, about 5 millimeters across, interrupted by a gelatinous barrier called the cupula. He started by treating the barrier as a perfect insulator, giving the proposed sensor its best possible chance.

For head-turning speeds reported in pigeons, the predicted voltage reached roughly 12 billionths of a volt, or 12 nanovolts. The voltage is tiny but not automatically disqualifying.

The real problem is noise: random thermal motion of charged particles inside the canal creates electrical fluctuations that overwhelm the magnetic signal. Filtering narrows the accepted frequency band and cuts some noise, but it also slows the response — and the pigeon's head keeps turning.

To act as a compass, the sensor must translate changes in its voltage into specific head directions, and it must do so quickly. Kattnig applied information theory, a framework for measuring how much data a noisy channel can carry, and found that the best-case model topped out near 0.15 bits per second.

Distinguishing directions 5 degrees apart during a rapid scan would require more than 560 bits per second. Allowing the cupula to leak current — closer to what real tissue does — pushes the performance further down. More sensitive sensory cells or heavier computation in the brain cannot recover directional information the sensor never delivered fast enough.

What remains unexplained?

The result leaves researchers with a puzzle. Magnetic fields consistently activate brain regions tied to the inner ear. Yet the induction mechanism that has dominated the discussion cannot, on paper, supply directional information quickly enough to keep a compass running.

Any alternative explanation will have to do two things at once: account for the consistent brain activation in the balance system during magnetic stimulation, and deliver a directional signal fast enough for the bird to navigate.

The biological observations from the 2019 Current Biology model and Nordmann's Science study remain standing as data. What Kattnig's analysis challenges is the induction-based physical interpretation of those observations, not the observations themselves.

The full paper, "Signals too small to sense: physical and information-theoretic limits to induction-based magnetoreception in birds," appears in the Journal of the Royal Society Interface. Its DOI is 10.1098/rsif.2026.0332.