how birds sense magnetic fields
=biology =physics =optics =explanation
It is known that many birds are able to sense the direction of Earth's
magnetic field.
Here's a wikipedia page on that general phenomenon. There have been 2
main theories of how that works.
One theory is that birds have
magnets in their beak that act like a compass. We know this is the correct
theory because:
- Small magnetite crystals have been found in bird beaks.
- Anaesthesia of bird beaks seems to affect their magnetic sense, sometimes.
The other theory is that birds have some sensing mechanism in their eyes that uses magneto-optical effects. We know this is the correct theory because:
- Birds can't sense magnetic field direction in red light.
-
Covering the right eye of birds prevents them from sensing field direction.
We also know those theories probably aren't both correct because:
- Most animals don't have a magnetic field sense. It's implausible that birds developed two separate and redundant systems for sensing magnetic fields when other animals didn't develop one.
organic magneto-optics
It's possible for magnetic fields to affect the optical properties of
molecules;
here's an example, a fluorescent protein strongly affected by a small
magnet. However, known examples of this require much stronger (~1000x)
fields than the Earth's magnetic field.
Let's suppose birds sense
magnetic fields using some proteins in their eyes that directly interact
with fields. The energy density of a magnetic field is proportional to the
field strength^2. The energy of interaction of a magnet with a field is
proportional to the product of the field strengths. The earth has a field of
25 to 65 μT. If we consider the energy of a strongly magnetic protein
interacting with the Earth's magnetic field, that's not enough energy to
directly cause a cellular signalling effect.
So, magnetic fields must
act to control some energy-transferring process, and the only logical
possibilities are light absorption/emission and transfer of excited states
between molecules. Birds can sense the direction of magnetic fields, more so
than field strength, so the effect of magnetic fields must be relative to
the orientation of something. Molecules are randomly oriented, but
absorption/emission of a photon is relative to molecule orientation, so
magnetic fields can create differences in absorption/emission of light at
different angles. (That's the basis of a spectroscopy technique I
previously proposed.)
For excited states of molecules to interact
with a magnetic field, they must have a magnetic field. The excited states
with the strongest fields would logically be triplet states, where the spin
of an electron is reversed, creating a net spin difference of 2. (The
magnetism of iron comes from the spin of its one unpaired electron, so
triplet states are more magnetic than iron atoms.)
Molecules
absorb/emit photons only of specific wavelengths: as energy and momentum are
conserved, molecules must have a vibrational mode that matches the photon.
Magnetic fields can shift what wavelengths are absorbed. Considering the
energy density of the Earth's magnetic field and the magnetic field of
triplet states, shifting the affected wavelengths of visible light by 1nm
seems feasible.
A blue sky doesn't seem to have sharp enough spectral
lines. Can one be made artificially? It's not normally possible to absorb a
wide spectrum of light and emit a narrow spectral line: thermodynamically, a
more narrow spectrum has a higher "temperature". The spectral width of
emission is typically about the same as the width of absorption. (This is why early
laser types are so inefficient: they only absorb a small fraction of the
light used to pump them. Systems using diode lasers are more efficient.) Thus, we need to absorb only a narrow
spectral line.
Fluorescence
can be delayed; see "phosphorescence". A
fluorescent molecule with a narrow spectrum would only absorb/emit a small
fraction of light, but supposing the emission is slightly delayed, birds
could blink their eyes and detect emitted light against a black background.
The emitted light could theoretically be detected by a separate
magnetically-sensitive molecule, but that would require evolving 2
molecules/proteins tuned to the exact same wavelength. A more likely
possibility is that the fluorescent emission rate itself is affected by
magnetic fields. A common mechanism for delayed fluorescence involves
triplet states, which create an interaction with magnetic fields. Supposing
the rate of light emission is affected by magnetic fields, it would be
faster in certain directions.
How could small shifts in the direction of emitted light be detected?
Supposing there's a pattern of magnetically-sensitive fluorescent pigment on
the surface of the eye, and a separate pattern of another fluorescent
pigment, the relative position of those patterns could be detected. This
would be a complex thing to detect, which would explain the somewhat long
acclimation times birds have to changes in magnetic field strength.
What would small dots on the surface of the eye look like? You've probably
seen the answer for yourself: when looking at a clear blue sky, people
can
often see white blood cells in blood vessels in the eye. Different
fluorescent pigments having changes in light emission with direction would
then be similar to such dots having some chromatic aberration that varies
with eye direction. Except, instead of dots, there might be more complex
patterns, and they would be visible when blinking. That does seem like
something birds could interpret, and something that could require a day of
adaptation when field strength changes.
the problem
Instead
of guessing, we can consider experiments on birds in artificial light. As
this paper
notes:
Tests under near-monochromatic lights revealed that orientation is possible under light from ultraviolet to about 565 nm green; under yellow and red light, birds are disoriented.
The above mechanism cannot work with a wide range of nearly-monochromatic light. Now, we've demonstrated how well we understand complicated optical effects, so let's try to actually find the answer.
magnetite
As I
mentioned, a tiny amount of magnetite has been found in bird beaks.
Researchers have also found that magnetic field changes
affect
nerves in the beak. Birds have some behavioral responses to magnetic fields
in the dark; they're just different and less directionally oriented than the
response when they can see through their right eye.
Some bacteria
have tiny magnetite crystals in them that cause them to orient along the
Earth's magnetic field. Magnetite is not usually a very strong magnet, but
tiny crystals of it only have a single domain, making them about as strong
as Nd magnets. (See
this post for an introduction to magnets.) As such, we know that a
freely-floating cell can act as a compass.
If we consider a cluster
of rod-like cells with narrow magnetite crystals, freely floating, they
would collectively produce more force, enough to be sensed by
mechanoreceptors.
compass
When humans
use a compass, they don't feel the movement of the compass needle,
they see it. Perhaps birds do the same.
Consider a cluster
of cells containing magnetite crystals. Now, put it on the surface of the
eye, and add sheets of different pigments along different planes. That would
cause its apparent color to change with its orientation. As mentioned above,
even single cells on the surface of the eye
can
be seen. Birds would then perceive magnetic field direction as changes
in the color of similar dots. The relaxation time and perhaps orientation
accuracy would depend on field strength, which could explain the adaptation
time to field strength changes. It's also possible that the clusters are
weakly connected to surrounding cells, such that their position would depend
on field strength as well as direction; logically, some sort of anchor to
keep the cell clusters near a consistent location would be needed.
two systems
The
current consensus (I think) among researchers studying bird magnetoreception
is that birds have 2 separate systems for sensing magnetic fields.
The same core structure (a cluster of cells with single-domain magnetite
crystals, mostly freely-floating) would be used in both above systems. That
makes evolution of 2 partly-redundant systems much easier.
Each system would
have some advantages. Mechanoreceptors would only detect the force applied
by each magnetic cell cluster, not the exact direction. Detecting direction
optically would be more accurate, but requires light, and would mainly
detect field direction, not strength.