Radio Noise Can Disorient Migrating Bats for Hours After Exposure

Bats have an internal compass, a fact scientists have known for some time. But researchers had largely assumed that the electromagnetic hum from cities had little effect on migrating animals. The thinking was that, if an animal passed through quickly, any interference with its navigation would be short-lived.

A new study suggests that assumption was not merely a little wrong: its findings could change how researchers understand the problem.

Soprano pipistrelle bats exposed to radio noise

The study focuses on the soprano pipistrelle, a tiny bat weighing roughly 6 grams. Every autumn, tens of thousands travel south along the Baltic coast, finding their own way through the night sky.

Richard Holland, professor of animal behaviour at Bangor University in Wales, helped lead the research with colleagues from Latvia and Germany.

The team wanted to find out whether the faint radio noise emitted by everyday devices disrupts a bat’s sense of direction. We are surrounded by this noise: it comes from power lines, electronic equipment and even LED bulbs.

Researchers caught wild bats and exposed them to weak broadband radio noise for approximately half an hour around sunset. They then released the animals and observed the direction in which each flew into the darkness. Bats that had not been exposed behaved as expected; those that had received the exposure did not.

Why sunset matters

Sunset was important to the experiment because of an unusual feature of these bats’ navigation. They appear to have an internal compass aligned with the Earth’s magnetic field, which they reset each evening using the point on the horizon where the sun sets.

There is evidence for this magnetic sense. An earlier study found that soprano pipistrelles respond to the angle at which the Earth’s magnetic field dips into the ground, a cue used by many birds as well. When scientists artificially tilted the field, the bats altered the direction in which they took off.

The team therefore had a clear prediction: interfere with the signal during the short sunset window when the compass is calibrated, and the bats should become disoriented. Expose them outside that window, and they should navigate normally. The prediction was straightforward to test, but proved only half right.

Lost either way

Bats exposed to radio noise lost their bearings and took off in all directions. Rather than travelling in the direction their migration required, they scattered. The unexposed bats oriented themselves correctly for the season, just as predicted.

The unexpected result was that the timing made no difference. Bats exposed well after sunset became just as disoriented as those exposed during the period thought to be critical for compass calibration.

This had not been observed before. The disruption was not confined to the moment when the compass is set. Radio noise appeared to disorient the bats whenever they encountered it, suggesting that the simple sunset explanation was incomplete.

Disorientation lasted hours after silence

The most surprising finding concerned what happened after the noise stopped, rather than when it was heard. The known effect of this type of noise on magnetic sensing is expected to disappear as soon as the signal ends. Once it is switched off, the animal should regain its bearings.

That was not what happened here. The bats remained confused for hours after the radio noise ceased, long after its source had been switched off and left behind. Researchers describe this as a carryover effect, which existing science had not predicted.

Holland found the result genuinely unexpected. His team had set out to investigate the bats’ magnetic sensing system, but found a more complicated picture than a simply jammed compass: a short exposure to noise had an effect that persisted beyond the signal itself.

Why bats wander

The cause of the lasting effect remains unknown, and the researchers are cautious about drawing firm conclusions. One possibility is that the noise makes the magnetic field seem unusual to a bat, causing it to stop trusting the field and disregard the cue altogether. That explanation has not been confirmed.

Another possibility is that the noise acts as a stressor, prompting a bat to abandon migration for the night. This could account for the aimless scattering instead of a confident departure in a particular direction. Both explanations remain possible.

The recordings reveal the behaviour: departures in random directions and an effect lasting hours. They do not reveal the mechanism behind it. Investigating that gap is the next stage of the work.

What this changes

For years, electromagnetic noise was assumed to be an urban problem that migrating animals could quickly leave behind. Under that view, a fast passage through the noise would cause no lasting harm.

The new work challenges that assumption. A short, weak exposure was enough to disorient these bats, and its effects lasted for hours rather than disappearing as soon as the animals flew clear.

That has implications beyond the experiment. Animals moving through landscapes filled with lights and wiring may carry the disruption with them long after passing its source, even if their exposure is brief.

Wildlife is left unprotected

Regulation raises a separate concern. Current exposure limits are based solely on human health, leaving wildlife unprotected even when those limits are met. There is also reason to think the findings may matter beyond this one bat species.

Soprano pipistrelles travel in a similar way to small songbirds that migrate at night. Such birds have been shown to lose their magnetic orientation when exposed to urban electromagnetic noise, and bats may encounter many of the same difficulties.

The noise from our devices may therefore reach further into the natural world than previously expected.