/ 19 July 2013

Unlocking the mystery of bird migration

A flock of migrating starlings flies over a field near the southern Israeli town of Netivot.
A flock of migrating starlings flies over a field near the southern Israeli town of Netivot. (AFP/David Buimovitch)

Winter is here and even Durban, where heat blurs the edges of things, has been honing itself against the coming season.

Inland, towards the mountains, kestrels perch in pairs on telephone wires and the swallows turn their circles through the whitening evening skies.

The Germans have a word for it, zugunruhe, migratory restlessness, a pulling.

When experiments are done on birds that are confined to cages during migration periods, they hop and flutter their wings for extended periods of time; periods that mirror the length of the actual migration and their bodies turn in the appropriate direction.

How birds migrate still remains a fascinating question and at the Centre for Quantum Technology at the University of KwaZulu-Natal it is being addressed on the most fundamental level.

That birds find their way by feeling the effects of the earth’s magnetic field is fairly well established, but how they negotiate this field and know which direction to fly in is less clear.

In terms of magnetic field navigation there are two dominant theories. One is the possible alignment of ferromagnetic crystals in the birds’ beaks.

For those familiar with high school science it is the same principle as iron filings forming patterns under the influence of a bar magnet.

Research suggests, however, that this system is limited to measuring the intensity of the field rather than the birds’ alignment with respect to the field.

The second theory, which is gaining traction, suggests instead that birds align themselves by exploiting quantum effects.

The quantum theory
Quantum theory sometimes creates the impression of a baffling and quasi-metaphysical world far removed from the commonplace phenomenon of bird migration.

Anyone who has tried to make sense of the Schrodinger’s cat thought experiment — where quantum theory allows for something to be in two different states at the same time, both dead and alive — experienced the difficulty of applying quantum propositions to the world on the scale of human experience.

Since it describes the interactions of microscopic particles in a manner that is counterintuitive to how the macroscopic reality of the world is perceived, quantum theory pushes the limits of understanding.

However, it is still a well-confirmed theory and proven by numerous experiments.

To imperfectly paraphrase the physicist Richard Feynman: nobody understands it, but it works.

There is a marked difference between the particles described by elementary quantum theory and the basic biological systems that are more common.

Elementary quantum theory describes the interactions of isolated particles at temperatures below -200°C (even the recent drop in Durban’s mercury can’t approximate such extremes) whereas living systems require higher temperatures and are not isolated.

One way in which quantum theory can be adapted to suit the purposes of biological systems is through the theory of open quantum systems which, as the “open” suggests, allows for interaction with the environment to be included in the dynamics of the system.

The role that quantum theory might play in migration can be understood through the “radical pair mechanism”.

An atom consists of protons, neutrons and electrons. The protons and neutrons are arranged in the densely packed nucleus of the atom, while the electrons orbit around the nucleus occupying different energy levels.

The further away the electron is from the centre of the nucleus the higher its energy level.

A radical pair occurs when a photon (light particle) transfers energy to an electron causing it to move to a higher energy level.

Because of the electron’s greater energy and increased distance from the nucleus, it becomes possible for it to overcome the attractive force exerted on it by the nucleus and ultimately be donated to a neighbouring atom or molecule.

Two electrons can occupy the same energy level in an atom if they are in different “spin” states.

When one electron is donated to a neighbouring molecule the result is an unpaired electron in both donor and acceptor molecules.

The radical pair begins its existence with correlated spins, which means that the electron spins are aligned in a particular way to each other.

Each electron in the radical pair interacts with the nuclei that surrounds it as well as with the earth’s magnetic field.

These interactions cause the electrons in the radical pair to change their spin with respect to each other.

The rate of this spin change depends on how the bird is orientated in the magnetic field, which means it can be used to measure this orientation.

The genius of the unknown
It’s not yet fully understood how these different spin states might translate into a signal that the bird can interpret, but it’s been suggested that they result in different chemical products, which in turn allows the bird to “see” in which direction it needs to fly.

Experiments suggest that this mechanism is in the bird’s right eye and a biological molecule, cryptochrome, has been put forward as governing the reaction.

Cryptochromes are flavoproteins that act as photoreceptors. Although scientists understand how cryptochrome behaves in plants, the possible role of this molecule in migration is far from certain.

There are still a number of riddles that need to be answered:
• What is the exact structure of the material that allows for this intricate interaction with the magnetic field?
• Why, for instance, do birds migrate perfectly in blue and green light but are disoriented in yellow and red light?
• What role might entanglement —the quantum correlation between particles despite their spatial separation that physicist Albert Einstein called “spooky action at a distance” — play in this mechanism?

That something as big as the earth might interact with something as small as an electron, and that a bird might be able to read the rainbow and translate light into a sense of direction is awe inspiring — in the least-tired use of that phrase.

Adams is a postgraduate student at the University of KwaZulu-Natal’s Centre for Quantum Technology