Migratory animals are truly remarkable navigators, they can successfully navigate over tens of thousands of kilometres – often at night or during poor weather conditions and using nothing more than their senses. What’s more many species can successfully perform these feats of navigational prowess with little or no help from more experienced individuals. These long-range navigational abilities have so far not been fully explained, and likely performed differently by different species using visual clues in the landscape, specific sounds, the position of the sun and stars and surprisingly the earth’s magnetic field itself. Even more remarkably recent research suggests that truly understanding how migratory birds specifically can sense the magnetic field around them requires an intersection of biology, chemistry and quantum physics!

Migration map of the White Stork based on Global Register of Migratory Species data. (image credit: user “Bamse” on wikimedia)

Possible European Robin migration patterns, the winter-only range is marked in blue, while the summer-only range is marked in yellow. Arrow indicate migration routes proposed by F. Packmor, et al.

The Geo-magnetic field

Anyone that has navigated using a compass knows just how magical of a tool it is. Simply adjust the declination to account for the magnetic and geological north pole not being aligned and you got an instrument that requires no modern technology – no satellites, no complicated inertial navigation system, no cellular networks- but can still measure North within a few degrees, assuming you are not near one of Earths poles that is, where it becomes next to useless.

And in that weakness of the compass lies the true strength of the magnetic field for navigation, earths magnetic field roughly resembles a bar magnet not only pointing North-South but also doing so at different angles depending on how far North or South along earths surface you are. Therefore, a compass does not worth near the poles since the field points nearly directly upwards or downwards breaking the compass! The needle which is trying to align itself with the direction of the magnetic field will simply spin in place since the compass is not designed to allow it to point upwards or downwards.

A diagram of Earth’s magnetic field (credit: Live Science)

An animal that can sense the magnetic field directly (referred to as magnetoreception) could use this to their advantage, feeling not just their north-south orientation but also their latitude based on the direction of the magnetic field.

Quantum Spin

Sadly, before we can discuss the birds, we need to take a quite detour to briefly discuss one of the crowning achieves of 20th century physics – quantum mechanics, a remarkably good theory for explaining physics when gravity is relatively weak (e.g., far away from supermassive objects like blackholes).

It does away with the continuous nature of classical physics instead requiring energy can only take of certain specific values (hence quantum mechanics – Max Planck borrow the term from Latin before changing its meaning, directly translated it means “how much of”, also giving use the term quantity). While quantum mechanics largely uses the same quantities (e.g. energy) as classical physics just requires they can only take on specific values, it also add some new quantities, for instance particles in quantum mechanics have a type of angular momentum called spin which for an electron can be either “up” or “down”.

Quantum mechanics is also inherently probabilistic, even in the simplest real life examples we can only describe what a particle will do in terms of likelihood of one outcome or another. What exactly the implications are of this probabilistic nature is generally very controversial among humans, and likely not discussed at all among migratory birds. The important take away is that whatever an electron has “up” or “down” is probabilistic but we can

Geomagnetic Quantum Birds

While biologists have found strong evidence for animals being able to sense magnetic fields until very recently there has been little evidence for how these animals can do that. Unlike electric sensing fish for instance which have well described specialised organs for generating and detecting electricity, a magnetic field sensing organ has not been found. Some fish are suspected to use a similar organ, but land animals can’t be due to their distinctly less watery (and hence less conductive) environment.

Iron can be magnetic and is required by all living things and is in fact found in large amounts in the blood of vertebrates (including humans!) in a protein called haemoglobin which gives vertebrate blood its characteristic red colour. However, there hasn’t been much evidence for it being involved with sensing earths rather weak magnetic field.

This left scientists with just one plausible hypothesis, a highly specialised protein containing two electrons that can either have aligned or anti-aligned spins switching between the two states at random. If there is no magnetic field the likelihood is equal for both options, however once there is a magnetic field that is no longer the case, and one outcome becomes more likely. Only proteins known as cryptochromes have ever been observed to have this property in planets when exposed to light, scientists followed up this lead and found similar proteins inside bird eyes.

However, that is not sufficient evidence on its own, and only recently after almost 20 years of active research a group at Oxford has been able to obtain pure samples of one of these proteins known as cryptochrome 4 (or CRY4 for short) and able to actually measure the molecule is magnetically sensitive.

Even more remarkably when the same protein was isolated from a domestic chicken (not known for migrating) and a European robin (which does migrate during winter months) they found that CRY4 proteins in chickens had a much weaker response to magnetic fields.

Other studies of European robins have found that they can only navigate using the magnetic field under blue or green light, which are the type of light required for the CRY4 protein providing further evidence of the hypothesis.

Perhaps the most interesting result so far is that if this is the mechanism by which birds can sense magnetic fields, they might actually see the fields. The proteins are in the birds’ eyes, and the brain area that we know is involved in magnetic navigation is located near ones involved with vision.

What seeing magnetic fields would be like we don’t know; I leave to the philosophers the question of what it’s like to be a European robin. There is some tentative evidence that robins have specialised receptors in their eyes adapted to see these magnetic fields so perhaps it is like seeing another primary colour to add to the regular cast of red, green blue, and ultraviolet (which birds can also see).