We have all heard about birds being able to sense the Earth’s magnetic field in order to navigate during migration, but how can they do that?  There have been a number of studies that implicated iron-rich cells in the upper beak of birds as magnetoreceptors.  Just like there are neurons that can sense mechanical stimuli, the idea was that the iron in these magnetosensitive neurons is affected by the Earth’s magnetic field in such a way that the neurons then become activated, signaling a change in the intensity of the field. 

There is no doubt that birds can sense a magnetic field, but a recent paper published online in Nature disputes that there are magnetosensitive neurons.  It’s unusual to find a paper based mainly on negative results published in a prestigious journal.  I like controversies in biology, so let’s take a look at this one.

First, the authors, Treiber et al., very systematically mapped the location of all the iron-rich cells in the beak.  You can find these cells by simply staining thin sections of the beak with a dye called Prussian blue, which labels ferric iron.  They found that the number and distribution of iron-positive cells was extremely variable from pigeon to pigeon of the same age and sex.  Sensing the magnetic field is very important to birds, so you would think there would be a near constant number and location of iron-rich cells, if these were in fact sensing the field.

In order for these cells to be acting as sensors, they would really need to be neurons and feed into the bird’s brain.  The authors stained the iron cells in the beak with markers for neurons and saw almost no overlap.  In other words, the iron-rich cells are not neurons.  Well, what are they then? 

Macrophages!
Here’s a macrophage engulfing some bad bacteria.
Macrophages are cells of the immune system.  They eat up bacteria and other pathogens and destroy them.  It turns out that macrophages also store iron that is released from hemoglobin when old red blood cells are recycled.  It makes complete sense, then, that the iron-rich cells in the beak could be macrophages just doing their normal storage job.  The iron-rich cells in the beak look like macrophages, but the authors confirm this by positively staining these cells for a macrophage-specific marker.  These cells are definitely not involved in sensing the magnetic field, since they can’t send signals to the brain.

This paper brings us back to square one: how do birds sense the Earth’s magnetic field?  All we know is that they don’t do it with the iron rich cells in their beaks.

UPDATE: Birds may not have magnetoreceptors in their beaks, but they do have neural correlates of magnetic fields in their brains.  In a recent report published in Science, researchers recorded from neurons in pigeon brains while varying a local magnetic field around them.  They found that individual cells in the brainstem would get activated when the magnetic field was pointing a particular direction.  Some neurons preferred 15 degrees, others 90 degrees, etc.  This is a strategy that the brain uses for other sensations, such as vision and audition.  Individual neurons are “tuned” to a particular stimuli.  We still don’t know how birds are able to sense magnetic fields, but now we know that their brains are set up to interpret that information coming in from the mystery receptors.

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