There are phenomena that people have known about for a long time, but it is impossible to know the reason or mechanism behind them. One of them is the way migratory birds remember their routes each year.

You know, those birds can navigate very well. In winter they move to the South to avoid the cold, and then return to the North when winter is over. The reason they can do so is thanks to their ability to navigate their way through the Earth’s magnetic field – at least that’s what scientists previously believed.

Birds’ ability to sense magnetic fields lies in a protein in their eyes

In other words, migratory birds have the ability to see magnetic fields. But the secret behind that ability cannot be deciphered until now.

Previously, science believed that bird beaks contained iron, so they could sense magnetic fields. But according to new research conducted by experts from Sweden and Germany, the secret lies in a protein in bird eyes, called Cry4.

Cry4 belongs to a subclass of proteins called cryptochromes – a type of light-sensing receptor that is very sensitive to blue light, found in both animals and plants. But in a study in April 2018, it was found that Cry4 seemed to be able to confirm the Earth’s magnetic field, and called it magnetoreception – or magnetic field sensing receptor.

In fact, birds’ ability to sense magnetic fields seems to only appear when light is there. In other words, it depends entirely on blue light from the Sun. This also means that this ability is related to vision, and birds really “see” magnetic fields.

To get this result, experts from Sweden’s Lund University and Germany’s Carl von Ossietzky University Oldenburg experimented on two species of birds. 1 is a zebra finch, and 2 is a European robin.

The team from Lund University identified three types of cryptochromes, Cry1, 2, and 4, in the eyes of the chimpanzee. The hypothesis is that these three proteins are responsible for the ability to sense magnetic fields.

Through some other testing, it seems that Cry1 and 2 have changed in density very significantly from day to day. Meanwhile, Cry4 is always kept at a stable level, and becomes the only candidate.

This is what birds can see

Research in Oldenburg also showed similar results. In addition, they made some very interesting discoveries. The first is Cry4, which is highly concentrated in the bird’s retina – the place that absorbs the most light. Meaning, that ability is completely dependent on light.

In short, birds really do see magnetic fields, and it’s all thanks to a type of light-sensing receptor called Cry4. However, science has not yet determined exactly what they actually see.

The research was published in the journal Current Biology.