Migratory birds like European Robins migrate throughout much of Europe, extending South to North Africa and the Mediterranean.
They travel vast distances to avoid the harsh winter.
To find the way, they have their three fundamental internal compasses-
•One which allows them to extract information from the position of sun in the sky.
•Second, which helps them to use the patterns of the stars at night.
•And the third one which is based on Earth’s ever present magnetic field, that is via magnetoreception.
To explain this, A German Biophysicist named, Klaus Schulten proposed an idea which states that the bird’s internal compass relies on magnetically sensitive chemical transformation.
But there is a major loophole in this idea-
The energy available from earth’s magnetic field is million times weaker than the energy needed to break or even significantly weaken the bonds between atoms in molecules and initiate a chemical reaction.
One way to explain this can be found from, Short lived chemical intermediates known as radical pairs have unique properties that make them chemically sensitive to feeble magnetic interactions.
This idea can be used to explain the ongoing process.
When photons from the light source hit a protein called CRYPTOCHROME4A located in the retinal cells of bird’s eye at a certain critical angle it creates a radical pair.
The two unpaired electrons, one in each radical, can have either antiparallel or parallel spins, known as singlet and triplet states.
Immediately after the creation of radical pairs, internal interaction causes the two electron spins to undergo a complex “quantum dance”.
This quantum dance in which singlet turns into triplet and vice versa a million times per second for a period up to few microseconds- The superposition of singlet and triplet states,
Until……………..
Under the right condition, the quantum dance can be influenced by the external magnetic field. This subtle quantum effect could form the basis of a magnetic compass sense that responds to the environmental stimuli a million times weaker than that would normally be thought possible.
Probability of being in one particular state with time when there is no magnetic field.
Probability of being in one particular state with time with magnetic field along a particular orientation.
Probability of being in one particular state with time when the magnetic field is rotated by 60 degrees.
From the graphs above we can see that both singlet and triplet states can undergo chemical reactions that transform them into the “signaling state”. The protein molecules return to the previous configuration and the process starts again. The proportion of outcomes depends on the bird’s orientation in earth’s magnetic field.
Now, signals continue to the bird’s brain where the magnetic information they contain is integrated with transformation from other directional cues, informing the direction of the bird’s flight. As an experimental verification, in 2007 Mouritsen and his colleagues ran an experiment in Oldenburg University by creating electro-smog, which supports the above theory.
So, next time when we see a migratory bird, we can now at least appreciate the underlying physics behind its flight.
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