How do billions of small migratory songbirds that fly thousands of kilometers every year between their breeding and wintering grounds find their way? They use celestial cues to navigate, much as sailors of yore used the sun and stars to guide them.
But unlike humans, they also have a “magnetic compass” in their eyes to detect the magnetic field generated by Earth’s molten core and use it to determine their position and direction.
According to a new discovery by researchers at the UK’s University of Oxford and Germany’s University of Oldenberg, while radio waves emitted by radio and TV broadcasting and citizens radio can interfere with this compass, those used in mobile communication networks do not because the frequencies are too high to affect their sense of orientation.
How humans interfere with birds' magnetic compass
The study has just been published in the journal Proceedings of the National Academy of Sciences (PNAS) under the title “Upper bound for broadband radiofrequency field disruption of magnetic compass orientation in night-migratory songbirds,” The research team was led by neurosensory sciences Prof. Henrik Mouritsen of Germany and chemistry Prof. Peter Hore of the UK.
This finding bolsters the researchers’ theory that the magnetic-compass sense in these birds is based on a quantum-mechanical effect known as radical pair mechanism in their eyes. For this study, the team combined behavioral experiments with complex quantum-mechanical calculations on a supercomputer.
Mouritsen, Hore, and colleagues had already proved nine years ago that electrosmog – human-made electromagnetic noise – in the AM radio waveband such as that generated by household electrical appliances, impairs migratory birds’ ability to use the Earth’s magnetic field for orientation, which is known as magnetoreception.
They suggested that this weak electrosmog, which is harmless for humans, affects the complex quantum-physical processes in certain cells in the retinas of migratory birds that enable them to navigate. But whether electrosmog also affects free-flying birds such as long-distance migratory birds, whose numbers have been declining for some time for unknown reasons, remained unclear.
The team took a closer look at the connection between the quantum-mechanical mechanism that they suspect forms the basis for the birds’ magnetic sense and the disruption of this mechanism by radio waves. The focus of their interest was the cut-off frequency above which the navigation of migratory birds remains unaffected, since determining this value allows conclusions to be drawn about the properties of the actual magnetic sensor in the birds. Their theory was that this sensor is a light-sensitive protein called cryptochrome-4 that has the necessary magnetic properties.
The scientists’ initial theoretical prediction was that the cut-off frequency would lie somewhere between 120 and 220 megahertz in the Very High Frequency (VHF) range, so the team conducted behavioral experiments with Eurasian blackcaps using different frequency bands within this range. In a study published in 2022 the researchers had already shown that radio waves of a frequency between 75 and 85 megahertz interfere with the magnetic compass sense of these small songbirds. These experiments showed that their magnetic compass stopped working when they were exposed to these radio frequencies but worked properly without exposure. Blackcaps are long and medium-distance migrants that can cover long distances during their annual migration
The researchers also performed model calculations in which they simulated the quantum-mechanical processes inside the cryptochrome protein. On the basis of these calculations, they were able to narrow down the cut-off frequency even further, to 116 megahertz.
Gaining a better understanding of magnetoreception is important for improving the protection of migratory birds, they wrote. It can also provide insights on key questions like what kind of electromagnetic radiation drives birds off course and should be avoided in areas like nature reserves where migratory birds stop to rest.