he Earth’s magnetic field is a dipole that acts like a large magnet, with its poles relatively near to the geographic (rotational) poles. Although the magnetic north pole is really in the geographic south position and vice versa, the magnetic north pole is typically referred to as the end of the dipole closest to the geographic north pole, and the magnetic south pole is similarly referred to as the end of the dipole closest to the geographic south pole. The geomagnetic field lines of force leave the magnetic South through Antarctica, circle the Earth, and re‑enter through the magnetic North’s surface, through the Arctic pole, creating vectors of these ascending lines of force in the Southern Hemisphere and descending lines of force in the Northern Hemisphere, which are parallel to the earth’s surface at the equator.
As one moves closer to the equator, the strength of the lines of force steadily declines, reaching maximum values of around 60,000 nT at the poles and around 30,000 nT at the Equator [1]. These characteristics make the magnetic field a very reliable and omnipresent source of information, in which the magnetic vector (the vector between the line of force of the magnetic field and the line of force of gravity) provides directional information that the bird can use as a “compass.” Further, the spatial distribution of other factors, such as intensity or inclination, can be components of the “map,” providing information on the geographic position of the bird as they vary between the poles and the equator. [2, 3].
A weak external magnetic field can modify the relative alignment of the electrons, thereby altering the reaction rate and yield, and is dependent on the intensity and the orientation of the external field.
For migratory birds, this process usually occurs on the surface of certain cell membranes. The direction of the external field lines produces arrays of protein oriented in the same direction, and the varying density of the synthesized protein enables the bird to determine the orientation of the field lines [5].
In order to produce these radicals, cryptochrome photoreceptors located in the eyes of the bird use photons from external light, such as the sun [4]. Behavior experiments determined that the photoreceptors and the bird’s ability to navigate are impacted by the wavelength of perceived light. Further, short wavelengths of light from UV to about 560nm were necessary for radial pair production [4]. Tests that involved birds in total darkness displayed a 90-degree shift in the preferred direction, which suggested that the radical pair sensing mechanism was not activated. Instead, a separate magnetic sensing mechanism behaves as a backup [5]. Although it is uncertain what the backup mechanism is, it likely relies on the magnetite-based magnetoreception mechanism.
Studies conducted on homing pigeons have determined that magnetite-containing dendrites are located at six locations on the upper beak. Clusters of the dendrites have been found to deform under weak magnetic fields, producing a torsion on the dendrite. We hypothesize that this torque behaves in the same manner as the radical pairs, and provides a complementary sensing method for magnetic fields [5].
Two coils placed in parallel along the same axis can produce a relatively uniform magnetic field between the coils, This configuration is known as a Helmholtz coil, named after the German physicist Hermann von Helmholtz. Three pairs of Helmholtz coils can be configured on the X, Y, and Z axis, allowing for complete control of the magnetic field in the center of the configuration.
A configuration of a 3D Helmholtz coil system enables the ability to cancel Earth’s magnetic field, as well as produce a static, rotating, or alternating field [6].
A similar experiment was conducted on homing pigeons [5]. It was determined that pigeons treated with pulses deviated from the untreated control group path, a deviation that became more substantial as the distance between home and the release point was increased. Overall, this indicated that the magnetic pulses would affect the internal avian compass but left the navigation map relatively unaffected, as the pigeons were still capable of returning home.
The following are two hypothesized magnetoreception models: By converting the strength of the Earth’s magnetic field into mechanical force within specialized cells, magnetoreception based on magnetite particles housed in the upper part of the beak function as chains of magnetite particles that interact with the Earth’s magnetic field and provide directional information and even geographic position to the birds.
Chemical magnetoreception based on a radical pair model, in which a molecule is energized by the absorption of a photon, produces an electron and forms a pair of radicals, which affects the speed of singlet-triplet interconversion depending on the alignment with the Earth’s magnetic field.
Because both magnetoreception models are hypothetical, the current understanding of magnetoreception models is insufficient to identify the magnetoreception model employed. Although a clear picture of how information from the magnetic compass is interpreted is beginning to appear, our present understanding of magnetoreception is constantly evolving.
There are still a lot of questions concerning magnetoreception and how data is processed from these receptors to the brain. Advances in behavior, anatomy, and physiology will aid in the discovery and identification of magnetic reception structures in the future.
- W. H. Campbell, Introduction to Geomagnetic Fields, 2003.
- R. Wiltschko and W. Wiltschko, “Avian navigation: From historical to modern concepts,” Animal Behaviour, vol. 65, no. 2, pp. 257–272, 2003.
- R. C. Beason, “Mechanisms of magnetic orientation in birds,” Integrative and Comparative Biology, vol. 45, no. 3, pp. 565–573, 2005.
- R. Wiltschko and W. Wiltschko, “Magnetoreception in birds,” Journal of The Royal Society Interface, vol. 16, no. 158, p. 20190295, 2019.
- M. Winklhofer, “The physics of geomagnetic-field transduction in Animals,” IEEE Transactions on Magnetics, vol. 45, no. 12, pp. 5259– 5265, 2009.
- N. Aldoumani, T. Kutrowski, J. Barnes, T. Meydan, and J. T. Erichsen, “Instrumentation to investigate the magnetoreception of homing pigeons by using applied magnetic fields,” IEEE SENSORS 2014 Proceedings, 2014.