How do migratory birds "see" the Earth's magnetic field with their eyes?

When the earth was first formed, it was still a molten ball with extremely high temperature, and various elements filled the entire sphere. As time went by, the sphere gradually "cooled down", and heavier elements such as iron and nickel slowly sank to the center of the earth, forming today's core.

Due to the extremely high temperature of the earth's core, all the metals here become liquid. When these liquid metals move, the speeds of the positive and negative charges in their bodies will be different, thereby generating electric current and then generating the earth's magnetic field.

It is because of the Earth's magnetic field that we are protected from the "love blow" of the solar wind. At the same time, the Earth's magnetic field also affects all kinds of creatures on Earth. The most important of these is the navigation information provided by the magnetic field.

As early as the Warring States Period, my country had an instrument called Sinan that used the Earth's magnetic field for navigation.

As for other creatures, especially migratory birds, scientists have long discovered that they also use magnetic fields as a means of navigation during migration.

But new questions arise: How do these birds sense the Earth's magnetic field to maintain the correct flying direction?

You should know that the magnetic field strength on the earth's surface is about 0.1~1 milligauss, which is very weak.

As the research continues to deepen, the secrets of birds have been "digged out" by scientists, and an important role in birds' sensing magnetic fields has also been found. It is cryptochrome, also known as Cry protein, which is a type of flavin protein that is sensitive to blue light.

The picture shows Cry1 protein

In 1880, Darwin recorded that blue light has a highly inhibitory effect on plant growth, but in the following hundred years, scientists did not find the substance that enables plants to sense blue light.

It wasn't until 1980 that researchers discovered that the HY4 gene in Arabidopsis thaliana was essential for plants to sense blue light.

In 1993, by sequencing the gene, it was discovered that the blue light receptor hidden in this gene was the Cry protein.

Soon after the discovery in plants, scientists were surprised to find that genes encoding Cry proteins also exist in the human body; later, in experiments on fruit flies and mice, cryptochrome genes were also discovered in these animals.

It was then that people realized that Cry protein is a light receptor protein widely present in eukaryotic organisms, and birds are no exception.

In animals, Cry proteins are mainly concentrated in nervous tissues, especially in tissues related to photosensitivity. For example, the retina is one of the most active tissues of Cry proteins. Obviously, Cry proteins are involved in all physiological activities regulated by light in animals.

In fact, it was discovered long ago that bird migration and navigation are also affected by light.

Because birds have a strong sense of direction when flying during the day but can easily get lost at night, the theory of geomagnetic perception alone cannot explain this phenomenon well. Therefore, some people have proposed that Cry protein is actually involved in the birds' sensing of the earth's magnetic field.

When the bird's eyes are exposed to light, the Cry protein forms a pair of free radicals with spin. According to the "Pauli exclusion" principle in quantum physics, electrons in the same orbit always have opposite spin directions.

If this state is affected by external factors, one of the electrons will be excited and "break away".

However, in Cry proteins, the two pairs of electrons can remain entangled for a long time after being excited. That is to say, no matter how far apart the two parties are, the behavior of one will affect the other. This is what Einstein called "spooky action at a distance."

In this way, the Earth's magnetic field begins its "performance".

When the pair of electrons are affected by the Earth's magnetic field, their spin state will change. However, because the Cry protein can maintain a "twisted" state for a long time, the electrons will not escape, but will return to the ground state orbit, and then continue to be excited by the Earth's magnetic field to change their spin state....

This ongoing interaction allows birds to continuously sense the Earth's magnetic field.

In addition, the CRY proteins at different locations in the retina sense different magnetic fields, which will cause differences in the activity of Cry proteins in different areas of the retina, thereby affecting the perception of light.

This is not a bad thing, however, because this difference in light sensitivity allows birds to know their current direction. So from the bird's perspective, their field of vision includes not only the scene they see, but also the difference in light and dark to help them determine the direction.

It is no exaggeration to say that birds can be said to have their own navigation system. This is something that people with poor directions would really envy.