Magnetic inclination Stop signs that birds use to identify their breeding places

Migratory birds have tremendous abilities to foresee their migration path based on the Earth's magnetic field. But how can she pinpoint their breeding sites with such accuracy?

The migratory bird does not find it difficult to find its flight path thanks to its ability to foresee the magnetic field of the Earth, in addition to its internal compass, which enables it to orient itself based on the strength of the magnetic field.

This biological sense, known as magnetoreception, allows birds to determine the direction of their migration, and enables them to return to their nests - which are often meters away from the site of their birth - and determine their return path very precisely.

The Earth is a huge magnet with magnetic north and south poles. These magnetic poles are located near the north and south geographic poles of the Earth. At the magnetic equator, the magnetic force of attraction is equal, which keeps the magnetic needle of the compass horizontal along this line without inclination or deviation, and therefore it is called the "non deviation line".

Hence, there are two properties of the magnetic field that birds may use in their migration:

• Magnetic inclination: It is the angle of deflection made by the magnetic needle between the Earth's surface and the magnetic field.

• Magnetic intensity: which represents the total strength of the magnetic field on Earth.

Variation of the magnetic field

But how do the birds know they have to stop because they have reached their destination? Do birds use the above signals to accurately determine their breeding sites?

A research study published in the journal "Science" on January 27 last provided an answer to this question. According to a press release published by Oxford University, the study was based on data from 17,799 Eurasian jungle warblers (Acrocephalus scirpaceus) that were tracked from 1940 to 2018. They are songbirds that migrate across the Sahara and use the magnetic field to return to places nesting;

Although these birds do use Earth's magnetic field signals to determine their original breeding site, the researchers believe that annual variations in the Earth's magnetic field must be followed by gradual changes in the birds' nesting sites.

And the authors of the study explain - in the report published on the "Science Alert" website - that "due to the change of the Earth's magnetic field year after year, the distinctive magnetic signs of the birth or reproduction site change successively in the following years." Thus the sites of birth or reproduction are located in different places.

"Thus, if birds use these magnetic markers to locate birthing or breeding sites, we would expect local changes in these sites from year to year," the authors add.

Magnetic inclination provides evidence

The results of the study indicated that magnetic inclination is the primary indicator that birds use to determine their breeding site, in addition to some specific tilt indicators that act as 'stop signs'. The study authors believe the birds "learn" the angle of inclination before leaving their breeding sites.

The scientists add, "Birds can call this coordinate dimension (the angle of inclination), which is a signal to stop, in addition to using their internal compass to infer their breeding and nesting sites.

Scientists believe that birds' use of magnetic inclination to determine locations is logical. According to the team, magnetic tilt is the most stable variable among other potential magnetic signals. Hence, it is the most reliable signal to the birds that they have reached their destination.

According to the team's estimates, the original birth site shifted an average of 18.5 km over those years. While the breeding site changed only 1.22 km, because the birds used the magnetic inclination as a signal to stop, which in turn enabled the birds to more accurately identify their breeding site.

In general, it seems clear that birds adapt a number of biological mechanisms related to the signals of the Earth's magnetic field to be able to navigate successfully and create suitable environments for them to live.