By India Today Web Desk: As the winter season began in India in late 2020, over 100 species of birds flocked from across the world to the Indian subcontinent in search of food and nestlings. These migratory birds fly thousands of kilometres across rivers, oceans and mountains, but how do they know the exact route and the spot every year?

New research says it's their ability to sense Earth's magnetic field that acts as a compass directing them in the long journey. Birds likely use magnetically sensitive proteins called cryptochromes located in their retinas that enable sensing and signalling functions, helping them in navigating these long distances.

The research published in the journal Nature brings us closer to solving the enigma of how migratory birds sense the change in weather and decide when to migrate and what route to take.

Researchers from the University of Oxford and the University of Oldenburg in Germany studied Robins and analysed the in-built "living compass" that they use to navigate. The team studied a form of the molecule to see if it has magnetic sensors and found that it had the capability of high magnetic sensitivity.

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"It looks possible - and I would put it no stronger than that at the moment - that these highly-specialised chemical reactions could give the bird information about the direction of the Earth's magnetic field and in that way constitute a magnetic compass," Professor PJ Hore of the University of Oxford told BBC.

While researchers are getting closer to detecting the inbuilt GPS, there have been other hypotheses over the years. Early research said that an oxidized-iron compound located in the body of the animals aligned with the magnetic field exerts a rotational force — called torque — which leads to changes in signals in body alignment and directing the movement.

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Another such hypothesis proposes that when cryptochrome proteins absorb photons of light and become ‘photoexcited’, they form magnetically sensitive chemicals known as radical pairs. Variations in the reaction signal the animal’s direction with respect to Earth’s magnetic field. Researchers say that there are chances that both these processes could happen in the birds that form their magnetic-map sense and magnetic-compass sense.

Previous studies on migratory Robins have shown that the Cryptochromes (CRY4) is located in the outer segments of two types of photoreceptor cell in the retina, which is an ideal location for receiving the light that would excite cryptochromes and thus aid magnetic sensing. Researchers also found that as the migratory season approaches, the expression level in Robin's retina rises.

The protein is also found in some animals and is known as CRY1 and CRY2 and regulate functions happening in a 24-hour cycle. Just like birds, CRY4 is also found in amphibians that have been documented to use the magnetic field to travel through the vast reaches of the oceans.

The team used a wide range of techniques, such as spectroscopic methods and molecular dynamics simulations, to reach the conclusion. However, they say that there is more work needed to be done to exactly decode this naturally occurring GPS in the birds.