How do bird eyes work

How do migratory birds find their way?

Many birds fly south in autumn and return in spring. It is now clear that migratory birds use the earth's magnetic field to navigate. They perceive this with their magnetic sense. Many questions are still unanswered, but the scientists have a candidate for the inner compass: a molecule on the animal's retina.

Every year, when it starts to get cold, the migration of birds can be seen in the sky. Many of the animals travel enormous distances to winter in warmer areas. The arctic tern holds the record. It flies once a year from Arctic to Antarctic regions - and covers around 15,000 kilometers in the process. Scientists assume that migratory birds orient themselves by the position of the sun. However, many of the animals cover large parts of their journey at night. In order to find the right way, they apparently use the earth's magnetic field.

The earth's magnetic field

"To navigate you always need two things: a map and a compass," explains Ilia Solov’yov from the University of Odense in the South of Denmark. The birds' magnetic sense is therefore made up of two components: On the one hand, the animals can detect the strength of the earth's magnetic field and, on the other, its orientation. Both skills are equally important. By recording the magnetic field strengths of different regions on their internal magnetic field map, the migratory birds can see where they have been before. “For example, if we move towards the equator, the magnetic field intensity changes slightly. There are also anomalies in the magnetic field near a large city. ”If the location is known, an internal compass helps the birds to determine the direction of flight.

For many years, scientists have been investigating where the magnetic sense is anchored in the animal's body. They suspect the inner compass to be in the eye of the birds. “We think that there is a certain protein on the retina,” says Solov’yov. “If you bring this protein into a magnetic field, its chemical properties change.” The researchers attribute this behavior to individual pairs of molecules in the protein. The molecules involved each have an excess electron that behaves like a small bar magnet. How the two tiny magnets align within a pair - parallel or antiparallel - depends heavily on the direction of the earth's magnetic field. And depending on how many of the pairs of molecules in the retina take on one or the other state, the chemical reactions in the eye take place slightly differently. In this way the bird can distinguish the cardinal points.

The researchers agree on the mechanism. However, it is not yet entirely clear which protein takes on the role of the magnetic sensor in the eye. The best candidate so far is a protein called cryptochrome, which biologists found in the retina of the bird's eye. Theoretical physicists then calculated how the protein would have to behave in a magnetic field. "We have shown that cryptochrome really has many great properties that can explain the compass magnetic sense," explains Solov’yov. “So far, however, there is no definitive proof.” An experiment is still missing, with which the researchers can show that cryptochrome in the bird's eye actually behaves as theoretically predicted - and shows the animal the direction.

Magnetic sensor in the bird's eye

The magnetic field map required for navigation is likely to be drawn by migratory birds using their beak. Here, scientists came across iron-containing particles that align themselves like small compass needles in the earth's magnetic field and could therefore be used as magnetic sensors. In the meantime, however, the iron particles have been found in unexpected places in the beak - in so-called phagocytes of the immune system. "These cells are responsible for removing the waste from the organism," says Solov’yov. "Under no circumstances can they lead to a magnetic sense."

It is uncertain whether the iron particles still play a role. “Something important is happening in the beak. We just don't know what, ”said Solov’yov. Scientists from various fields - quantum physicists, biologists and ornithologists - look for new knowledge together. The research is not only relevant for migratory birds. It is now known that many mammals, reptiles and insects also have a magnetic sense.