How do birds orient themselves during migrations? Scientists propose an answer

Many bird species are capable of covering thousands of kilometers without error during seasonal migrations or returning home after long flights. Decades of experiments have shown that they use not only the Sun, stars, landscape features, and even smells, but also the Earth's magnetic field for this. However, the mechanism of such navigation remained one of biology's most complex puzzles.

For decades, scientists have put forward various hypotheses regarding where exactly birds' "magnetic compass" is located. Possible candidates included cells in the eyes, inner ear, and beak. However, there was no conclusive evidence supporting any of these versions.

Liver Macrophages

As reported by the Financial Times, a group of researchers from the University of Bonn and the Max Planck Institute for Animal Behavior in Germany proposed a new explanation.

In their study, scientists focused on macrophages — immune system cells located in the liver. Their main function is to process old red blood cells. In doing so, these cells accumulate iron, making them particularly sensitive to changes in the magnetic field.

Initially, researchers scanned the bodies of pigeons to identify organs with the most pronounced magnetic response, and the liver demonstrated the strongest magnetic response. Further analysis showed that macrophages containing the protein ferritin play a key role. This protein is rich in iron and can behave like a kind of nanomagnet.

To verify their findings, researchers conducted an experiment with homing pigeons. Some birds were injected with clodronate, a drug that temporarily depletes the liver of macrophages. After this, scientists tracked the flight routes.

It was found that in cloudy weather, pigeons after such a procedure lost their orientation and flew chaotically. However, on clear days, they still successfully found their way, using the Sun as a navigational landmark.

Additional studies using electron microscopy showed that nerve fibers run close to macrophages in the liver. This could explain how information about the magnetic field is transmitted to the brain.

The authors of the work believe that they have obtained the first direct evidence of the existence of a mechanism through which the body can perceive the Earth's magnetic field and use this information for navigation.

Cryptochrome in the Retina

At the same time, as Financial Times notes, the question of how birds perceive the Earth's magnetic field has not yet received a definitive answer. Other researchers continue to study alternative mechanisms of magnetic navigation. For example, a group of scientists from Oxford University is investigating the hypothesis that quantum processes in birds' eyes may play a key role in this process.

This version is related to the cryptochrome protein, which is found in the retina of the eye. Under the influence of light, it promotes the formation of free radicals — short-lived molecules with unpaired electrons. Their reactions depend on the strength and direction of the magnetic field, and signals from these processes can be perceived by nerve cells in the retina. According to this hypothesis, birds can, in a sense, "see" the magnetic field.

Scientists not involved in the German study note that both hypotheses do not necessarily exclude each other. It is possible that birds use several different mechanisms simultaneously: one helps to orient during long migrations, while another helps to find the exact destination at the final stage of the journey.

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