Scientists in India have demonstrated through new and robust statistical techniques that changes in the Earth’s magnetic field guide birds along migratory pathways, bolstering evidence for a phenomenon known since the 1960s.

The scientists at the Indian Statistical Institute, Calcutta, and Visva-Bharati, Santiniketan, have generated fresh evidence to show how magnetic cues influence subtle directional changes every few hours in birds as they fly hundreds to thousands of kilometres to their migratory destinations.

“We’re calling them magnetic murmurs — mild and smooth changes in the magnetic field like whispers in the birds’ brains, urging them to change direction,” said Debashis Chatterjee, an assistant professor at Visva-Bharati who supervised the research.

Chatterjee and his colleagues analysed the navigational movements of six species — the black-bellied plover, black-crowned night heron, brown pelican, long-billed curlew, Pacific loon, and Swainson’s hawk — and the Earth’s magnetic field measurements along their migratory pathways across the American continents.

Their study has been published in the journal PLOS One.

“We picked these six species because their migratory pathways have been well documented in the past by ornithologists,” said Prithwish Ghosh, a study team member, currently a research scholar at the North Carolina State University in the US. “The statistics techniques we’ve adopted allow us to analyse the angular changes in directions with far greater accuracy than achieved before.”

Their study has revealed, for instance, that Swainson’s hawks that migrate southward along the Pacific coast make fine changes in direction every three hours to every 21 hours. Each such change in direction correlates with changes in the magnetic field, taking into account other factors such as strong winds or storms that might also influence their flight paths, said Chatterjee.

The first experimental evidence for the link between bird migration and magnetism emerged in the 1960s when German researchers Wolfgang Wiltschko and Roswitha Wiltschko found that European robins could detect and respond to changes in magnetic fields, suggesting that birds have a magnetic sense.

Since then, scientists have also identified possible biological mechanisms, including proteins in birds’ eyes that they believe play a key role in avian navigation. Scientists say birds’ capacity to use such an internal magnetic compass is innate, or programmed in their genes.

A study led by biologist Miriam Liedvogel at the University of Oldenburg, Germany, earlier this year added new evidence for the idea — first proposed in 2021 — that a protein called cryptochrome-4 in birds’ eyes is the magnetoreceptor underlying the magnetic field-guided navigation.

Liedvogel and her colleagues analysed the genomes of 363 bird species and found signs of what they have proposed are evolutionary changes in the proteins in migratory and non-migratory birds. The protein is much more sensitive to magnetic fields in robins — a migratory species — than in chickens. In their 2024 study, German researchers found that birds — such as parrots and hummingbirds — have lost the protein.

The ISI-Visva-Bharati team applied tools from directional statistics, a field of statistics that is appropriate for analysis of data involving angles and directions. A migratory bird’s pathway is like a giant arc that follows the Earth’s curvature.

“This makes directional statistics ideal to study bird migration,” said Chatterjee, who says he was inspired to take up research on bird navigation by the late Sarbani Patranabis-Deb, a former faculty member in ISI’s geophysics division. ISI scientists Amlan Banerjee and Shiladri Das are co-authors of the study.